Method of making a multi-electrode double layer capacitor having single electrolyte seal and aluminum-impregnated carbon cloth electrodes

ABSTRACT

A method of making a double layer capacitor consists of the steps of: impregnating each of a plurality of carbon preforms with a metal; forming a plurality of current collector foils, each of the plurality of current collector foils having a tab portion and a paddle portion; forming a plurality of electrodes by positioning one of the plurality of carbon preforms against respective paddle portions of each of the plurality of current collector foils, wherein each of the plurality of electrodes comprises one of the plurality of current collector foils and one of the plurality of carbon preforms; stacking each of the plurality of electrodes such that tab portions of adjacent ones of the plurality of current collector foils are offset, thereby forming an electrode stack; interposing respective porous separator portions between each of the plurality of electrodes, wherein the porous separator portions function as electrical insulators between the adjacent ones of the plurality of electrodes preventing electrical shorting against each other; applying a modest constant pressure against the electrode stack; saturating the electrode stack with an electrolytic solution; and maintaining the electrode stack immersed within the electrolytic solution.

This application is a Divisional Application of application Ser. No.09/377,327 entitled MULTI-ELECTRODE DOUBLE LAYER CAPACITOR HAVING SINGLEELECTROLYTE SEAL AND ALUMINUM IMPREGNATED CARBON CLOTH ELECTRODES, ofFarahmandi, et al., filed Aug. 18, 1999, now U.S. Pat. No. 6,233,135,which is a Continuation-in-Part of application Ser. No. 09/087,471entitled MULTI-ELECTRODE DOUBLE LAYER CAPACITOR HAVING SINGLEELECTROLYTE SEAL AND ALUMINUM IMPREGNATED CARBON CLOTH ELECTRODES, ofFarahmandi, et al. filed May 29, 1998, now U.S. Pat. No. 5,907,472,which is a Divisional Application of application Ser. No. 08/726,728,filed Oct. 7, 1996, now U.S. Pat. No. 5,862,035; which is aContinuation-In-Part of U.S. patent application Ser. No. 08/319,493,filed Oct. 7, 1994, now U.S. Pat. No. 5,621,607, all of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to an electric double layercapacitor, and more particularly to a high performance double layercapacitor made with low-resistance aluminum-impregnated carbon-clothelectrodes and a high performance electrolytic solution.

Double layer capacitors, also referred to as electrochemical capacitors,are energy storage devices that are able to store more energy per unitweight and unit volume than traditional capacitors. In addition, theycan typically deliver the stored energy at a higher power rating thanrechargeable batteries. Double layer capacitors consist of two porouselectrodes that are isolated from electrical contact by a porousseparator. Both the separator and the electrodes are impregnated with anelectrolytic solution. This allows ionic current to flow between theelectrodes through the separator at the same time that the separatorprevents an electrical or electronic (as opposed to an ionic) currentfrom shorting the cell. Coupled to the back of each of the activeelectrodes is a current collecting plate. One purpose of the currentcollecting plate is to reduce ohmic losses in the double layercapacitor. If these current collecting plates are non-porous, they canalso be used as part of the capacitor seal.

Double layer capacitors store electrostatic energy in a polarized liquidlayer which forms when a potential exists between two electrodesimmersed in an electrolyte. When the potential is applied across theelectrodes, a double layer of positive and negative charges is formed atthe electrode-electrolyte interface (hence, the name “double layer”capacitor) by the polarization of the electrolyte ions due to chargeseparation under the applied electric field, and also due to the dipoleorientation and alignment of electrolyte molecules over the entiresurface of the electrodes.

The use of carbon electrodes in electrochemical capacitors with highpower and energy density represents a significant advantage in thistechnology because carbon has a low density and carbon electrodes can befabricated with very high surface areas. Fabrication of double layercapacitors with carbon electrodes has been known in the art for quitesome time, as evidenced by U.S. Pat. No. 2,800,616 (Becker), and U.S.Pat. No. 3,648,126 (Boos et al.).

A major problem in many carbon electrode capacitors, including doublelayer capacitors, is that the performance of the capacitor is oftenlimited because of the high internal resistance of the carbonelectrodes. This high internal resistance may be due to several factors,including the high contact resistance of the internal carbon-carboncontacts, and the contact resistance of the electrodes with a currentcollector. This high resistance translates to large ohmic losses in thecapacitor during the charging and discharge phases, which losses furtheradversely affect the characteristic RC (resistance×capacitance) timeconstant of the capacitor and interfere with its ability to beefficiently charged and/or discharged in a short period of time. Thereis thus a need in the art for lowering the internal resistance, andhence the time constant, of double layer capacitors.

Various electrode fabrication techniques have been disclosed over recentyears. For example, the Yoshida et al. patent (U.S. Pat. No. 5,150,283)discloses a method of connecting a carbon electrode to a currentcollector by depositing carbon powder and other electricalconductivity-improving agents on an aluminum substrate.

Another related approach for reducing the internal resistance of carbonelectrodes is disclosed in U.S. Pat. No. 4,597,028 (Yoshida et al.)which teaches that the incorporation of metals such as aluminum intocarbon fiber electrodes can be accomplished through weaving metallicfibers into carbon fiber preforms.

Yet another approach for reducing the resistance of a carbon electrodeis taught in U.S. Pat. No. 4,562,511 (Nishino et al.) wherein the carbonfiber is dipped into an aqueous solution to form a layer of a conductivemetal oxide, and preferably a transition metal oxide, in the pores ofthe carbon fibers. Nishino et al. also discloses the formation of metaloxides, such as tin oxide or indium oxide by vapor deposition.

Still another related approach for achieving low resistance is disclosedin U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963 (Tatarchuk etal.). The Tatarchuk et al. patents demonstrate that metal fibers can beintermixed with a carbon preform and sintered to create a structurallystable conductive matrix which may be used as an electrode. TheTatarchuk et al. patents also teach a process that reduces theelectrical resistance in the electrode by reducing the number ofcarbon-carbon contacts through which current must flow to reach themetal conductor. This approach works well if stainless steel or nickelfibers are used as the metal. However, applicants have learned that thisapproach has not been successful when aluminum fibers are used becauseof the formation of aluminum carbide during the sintering or heating ofthe electrode.

Another area of concern in the fabrication of double layer capacitorsrelates to the method of connecting the current collector plate to theelectrode. This is important because the interface between the electrodeand the current collector plate is another source of internal resistanceof the double layer capacitor, and such internal resistance must be keptas low as possible.

U.S. Pat. No. 4,562,511 (Nishino et al.) suggests plasma spraying ofmolten metals such as aluminum onto one side of a polarizable electrodeto form a current collector layer on the surface of the electrode.Alternative techniques for bonding and/or forming the current collectorare also considered in the '511 Nishino et al. patent, includingarc-spraying, vacuum deposition, sputtering, non-electrolytic plating,and use of conductive paints.

The previously-cited Tatarchuk et al. patents (U.S. Pat. Nos. 5,102,745,5,304,330, and 5,080,963) show the bonding of a metal foil currentcollector to the electrode by sinter bonding the metal foil to theelectrode element.

U.S. Pat. No. 5,142,451 (Kurabayashi et al.) discloses a method ofbonding the current collector to the surface of the electrode by a hotcuring process which causes the material of the current collectors toenter the pores of the electrode elements.

Still other related art concerned with the method of fabricating andadhering current collector plates can be found in U.S. Pat. Nos.5,065,286; 5,072,335; 5,072,336; 5,072,337; and 5,121,301 all issued toKurabayashi et al.

It is thus apparent that there is a continuing need for improved doublelayer capacitors. Such improved double layer capacitors need to deliverlarge amounts of useful energy at a very high power output and energydensity ratings within a relatively short period of time. Such improveddouble layer capacitors should also have a relatively low internalresistance and yet be capable of yielding a relatively high operatingvoltage.

Furthermore, it is also apparent that improvements are needed in thetechniques and methods of fabricating double layer capacitor electrodesso as to lower the internal resistance of the double layer capacitor andmaximize the operating voltage. Since capacitor energy density increaseswith the square of the operating voltage, higher operating voltages thustranslate directly into significantly higher energy densities and, as aresult, higher power output ratings. It is thus readily apparent thatimproved techniques and methods are needed to lower the internalresistance of the electrodes used within a double layer capacitor andincrease the operating voltage.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing ahigh performance double layer capacitor having multiple electrodeswherein the multiple electrodes are made from activated carbon that isvolume impregnated with aluminum in order to significantly reduce theinternal electrode resistance by decreasing the contact resistancebetween the activated carbon elements.

In one embodiment, the present invention can be characterized as adouble layer capacitor, and method of making the same, comprising acapacitor case having a first part and a second part fastenable to eachother to form a sealed capacitor case. The sealed capacitor case has afirst capacitor terminal and a second capacitor terminal associatedtherewith. Also an electrode stack is contained within the sealedcapacitor container. The electrode stack comprises a plurality ofelectrodes, each electrode includes a current collector foil and acarbon cloth impregnated with a specified metal in direct physicalcontact with the current collector foil. The current collector foils ofalternating electrodes are coupled to the first capacitor terminal andthe current collector foils of other alternating electrodes are coupledto the second capacitor terminal. A porous separator material ispositioned between each electrode of the electrode stack. The porousseparator material has pores therein through which ions may readilypass. The porous separator material prevents adjacent electrodes fromelectrically contacting each other. The electrode stack is maintainedunder a constant modest pressure within the sealed capacitor case. And aprescribed electrolytic solution is sealed within the sealed capacitorcase, whereby the electrode stack is saturated and immersed within theelectrolytic solution. In one embodiment, the porous separator materialcomprises a contiguous porous separator sheet that winds in between eachelectrode of the electrode stack in a serpentine manner.

In another embodiment, the present invention can be characterized as awrapped electrode stack, and method of making the same comprising aplurality of impregnated carbon cloths, each having been impregnatedwith a specified metal; a plurality of current collector foils, eachhaving a tab portion and a paddle portion; and a plurality ofelectrodes, each electrode comprising one of the plurality of currentcollector foils making direct contact with one of the plurality ofimpregnated carbon cloths. An electrode stack comprises the plurality ofelectrodes stacked such that alternating tab portions align with eachother, forming a first set of aligned tab portions and a second set ofaligned tab portions. And a contiguous porous separator sheet windsthroughout the electrode stack in a serpentine manner between each ofthe plurality of electrodes and wrapped around the electrode stack, suchthat the contiguous porous separator sheet acts as an electricalinsulator between adjacent electrodes of the electrode stack.

In a further embodiment, the present invention can be characterized as adouble layer capacitor, and method of making the same. The double layercapacitor includes a capacitor case comprising a first part and a secondpart fastenable to each other to form a sealed capacitor case. Thesealed capacitor case has a first capacitor terminal and a secondcapacitor terminal associated therewith. The double layer capacitorincludes a first electrode comprising a first current collector foil anda first carbon cloth impregnated with a specified metal. The firstcurrent collector foil has a first tab portion and a first paddleportion. The first carbon cloth makes direct physical contact with thefirst paddle portion and the first tab portion is coupled to a firstcapacitor terminal. Also included is a second electrode comprising asecond current collector foil and a second carbon cloth impregnated withthe specified metal. The second current collector foil has a second tabportion and a second paddle portion. The second carbon cloth makesdirect physical contact with the second paddle portion and the secondtab portion is coupled to a second capacitor terminal. The firstelectrode and the second electrode are placed against each other,wherein a porous separator material separates the first electrode fromthe second electrode. The porous separator material wraps around thefirst electrode and the second electrode and acts as an electricalinsulator between the first and second electrodes. The first electrodeand the second electrode are compressed against each other with a modestconstant pressure within the sealed capacitor case. And a prescribedelectrolytic solution is sealed within the sealed capacitor case tosaturate and immerse the first electrode, the second electrode, and theporous separator material with the prescribed electrolytic solution.Again, in one embodiment, the porous separator material may be acontiguous porous separator sheet that winds in between the first andsecond electrodes in a serpentine manner.

In yet another embodiment, the present invention can be characterized asa method of applying a modest constant pressure to an electrode stackcomprising first providing an electrode stack. The electrode stackcontains a plurality of electrodes, each electrode having a currentcollector foil and a metal impregnated carbon cloth placed thereagainst,and a contiguous porous separator sheet that winds throughout theelectrode stack in a serpentine manner. A shim is placed against anexterior of the electrode stack, the shim having a specified thickness.And the electrode stack and the shim are inserted into a container, thecontainer having an interior dimension less than the exterior dimensionof the electrode stack having the shim placed thereagainst.

In yet another further embodiment, the present invention can becharacterized as a method of ultrasonically bonding multiple foilstogether to form an electrical interconnection, and electricalinterconnection formed, including the steps of: stacking a plurality ofmetal foils to be bonded together, each of the plurality of metal foilsbeing coupled to an electrical device; positioning at least one dummymetal foil against the plurality of metal foils; and using a highfrequency horn for ultrasonically bonding the plurality of metal foilsand the at least one dummy metal foil together, the high frequency hornbeing directed at the least one dummy metal foil, wherein the pluralityof metal foils remain intact and are bonded to each other and the atleast one dummy metal foil.

In still another embodiment, the present invention can be characterizedas a carbon cloth electrode for use in a capacitor comprising a carboncloth comprising a plurality of twisted carbon fiber bundles that arewoven together to form the carbon cloth. Each of the plurality oftwisted carbon fiber bundles comprises a plurality of carbon fiberbundles which comprise a plurality of carbon fibers. Each of theplurality of carbon fiber bundles are twisted together such that anexterior of each of the plurality of carbon fiber bundles slightly fraysto form the respective ones of the plurality of twisted carbon fiberbundles. The use of the plurality of twisted carbon fiber bundlesreduces the transverse resistance of the carbon cloth electrode.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings and Appendix, wherein:

FIG. 1 is a sectional view of a single cell high performance doublelayer capacitor made in accordance with the present invention;

FIG. 2A is a sectional representation of a bipolar aluminum/carboncomposite electrode made in accordance with the invention;

FIG. 2B illustrates an upper portion of a bipolar stack of FIG. 2A;

FIG. 2C illustrates a lower portion of a bipolar stack of FIG. 2A;

FIG. 3 is a sectional representation of a series bipolar stack of highperformance bipolar type double layer capacitors of the type shown inFIG. 2A;

FIG. 4A schematically shows a basic double layer capacitor made inaccordance with one embodiment of the invention;

FIG. 4B conceptually illustrates the activated carbon fibers that formpart of the carbon cloth used in the electrodes of the double layercapacitor, and additionally helps illustrate how a double layercapacitor is able to achieve such a large surface area, and hence alarge capacitance;

FIG. 5 shows the equivalent circuit diagram of the basic double layercapacitor of FIGS. 4A and 4B;

FIG. 6A shows a more detailed representation of the equivalent circuitdiagram of FIG. 5, particularly illustrating a relationship betweenelectrode resistance and electrolyte solution resistance;

FIG. 6B conceptually shows the alternate paths ions may take as thecurrent flows through a single electrode to illustrate resistance andcapacitance at various points in the electrode in accordance with oneembodiment of the present invention;

FIG. 7 is a simplified electrical equivalent circuit that illustratesthe role the internal resistance of the capacitor, R_(z), plays inefficiently delivering energy to a load;

FIGS. 8A and 8B schematically show one technique that may be used towire arc spray an activated carbon cloth with aluminum, therebyimpregnating aluminum into the tows of the carbon fiber bundles of thecloth, as illustrated in FIGS. 9A-9B;

FIG. 8C schematically shows a jet spray technique that may be used towire arc spray an activated carbon cloth with aluminum, as illustratedin FIGS. 9C-9D;

FIG. 9A shows a schematic representation of a side sectional view of thecarbon cloth, and illustrates how a plurality of fiber bundles are wovento form the carbon cloth;

FIG. 9B conceptually illustrates a cross-sectional view of an individualfiber bundle of the carbon cloth, and further conceptually illustrates apreferred penetration of the aluminum deep into the tow of the fiberbundle;

FIG. 9C shows a schematic representation of a side sectional view of thecarbon cloth made out of twisted carbon fiber bundles, and illustrateshow a plurality of twisted fiber bundles are woven to form the carboncloth;

FIG. 9D conceptually illustrates a cross-sectional view of a tripletwisted carbon fiber bundle of the carbon cloth, and furtherconceptually illustrates a preferred penetration of the aluminum deepinto the tow of the fiber bundle;

FIGS. 10A-10E illustrates a method for making a plurality of electrodesfor use in a multi-electrode double layer capacitor according to the“prismatic” design embodiment of the present invention;

FIG. 11 is a side view of a wrapped electrode stack or winding assemblymade from the electrodes in FIG. 10E illustrating the porous separatormaterial winding throughout the electrode stack in a serpentine fashionin accordance with one embodiment of the present invention;

FIG. 12 is a front end view of a winding assembly of FIG. 11, furtherillustrating the porous separator material winding throughout theelectrode stack in a serpentine fashion;

FIG. 13 is a top view of a winding assembly of FIG. 11, illustrating theorientation of the tab portions in the winding assembly in accordancewith one embodiment of the present invention;

FIGS. 14A and 14B are a side view and an end view, respectively, of abrick assembly made from the winding assembly of FIG. 11;

FIG. 15 is a shim that is used in the construction of the brick assemblyof FIG. 14;

FIGS. 16A and 16B show a front and side view, respectively, of acapacitor container or can used to hold the brick assembly of FIG. 14;

FIG. 17 is a side view illustrating how the brick assembly of FIG. 14 isinserted into the capacitor container of FIGS. 16A and 16B;

FIGS. 18A and 18B are a side and top view, respectively, of the brickassembly of FIG. 14 having been compressed inside the capacitorcontainer of FIGS. 16A and 16B;

FIGS. 19A and 19B are a side and top view, respectively, illustratingthe formation of the capacitor terminals from the tab portions extendingfrom the device of FIGS. 18A and 18B;

FIGS. 19C and 19D show a representation of an ultrasonic bondingtechnique used for bonding many current collector foils together;

FIGS. 20A and 20B are a front and side view, respectively, of the lidused to close and seal the capacitor case containing the brick assemblyof FIGS. 19A and 19B;

FIGS. 21A and 21B are a side view and a front end view, respectively, ofthe capacitor container of FIGS. 19A and 19B having been sealed with thelid of FIGS. 20A and 20B;

FIGS. 22A and 22B are a flowchart that illustrates the method of makingand assembling the preferred embodiment for the “prismatic” designdouble layer capacitor shown in FIGS. 10A-21;

FIGS. 23A and 23B show current-voltage graphs of the double layercapacitor made in accordance with one embodiment of the presentinvention, and further illustrate the working voltage obtainable withsuch design for two different levels of impurities (water) in theelectrolytic solution;

FIGS. 24A-24F illustrate a method for making a stack of electrodes foruse in a multi-electrode double layer capacitor according to oneembodiment of the present invention; for example, the “clamshell”design;

FIG. 25A illustrates how the individual electrodes of two electrodestacks, made as illustrated in FIGS. 24A-24F, one stack of which has aporous separator positioned against each electrode as shown in FIG. 24F,are interleaved to form an electrode assembly;

FIG. 25B depicts the electrode assembly of FIG. 25A after it is wrappedwith a suitable insulator material to form an electrode package;

FIG. 25C depicts an alternate spiral wound configuration of theelectrode assembly;

FIG. 26 is an exploded view of a “clamshell” double layer capacitor,illustrating how the electrode package of FIG. 25B is positioned insideof upper and lower conductive shells, which shells are tightly sealedone to the other to complete the capacitor assembly;

FIGS. 27A, 27B and 27C illustrate top, end, and end-sectional views,respectively, of an alternative capacitor case which may use either aconductive or a non-conductive case having capacitor terminals at eachend of the case;

FIGS. 28A and 28B are a flow chart that illustrates the method of makingand assembling the “clamshell” double layer capacitor shown in FIG. 24Athrough FIG. 26; and

FIG. 29 depicts current and voltage waveforms associated with testing adouble layer capacitor made in accordance with FIGS. 22A, 22B, 28A, and28B.

Appendix A sets forth the presently-used acceptance test procedures totest the performance of a capacitor after fabrication and assembly inaccordance with FIGS. 24A through 26 and 28A and 28B.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the presently contemplated best mode ofpracticing the invention is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles of theinvention. The scope of the invention should be determined withreference to the claims.

Referring to FIG. 1, a single cell, high performance double layercapacitor 10 is illustrated including a cell holder 11, a pair ofaluminum/carbon composite electrodes 12 and 14, an electronic separator18, an electrolyte 20, a pair of current collector plates 22 and 24, andelectrical leads 28 and 29, extending from the current collector plates22 and 24.

The pair of aluminum/carbon composite electrodes 12 and 14 arepreferably formed from a porous carbon cloth preform or carbon paperpreform which is impregnated with molten aluminum. The invention asdescribed is not limited to using molten aluminum, and can use anothersuitable metal, such as copper or titanium. The porosity of thealuminum/carbon composite electrodes 12 and 14 must be closelycontrolled during the impregnation process to subsequently permit asufficient amount of the electrolyte 20 to be introduced into the doublelayer capacitor 10 and penetrate the pores of the carbon fibers.

The pair of current collector plates 22 and 24 are attached to the backof each aluminum/carbon composite electrode 12 and 14. Preferably, thecurrent collector plates 22 and 24 are thin layers of aluminum foil.

An electronic separator 18 is placed between the opposingaluminum/carbon composite electrodes 12 and 14. The electronic separator18 is preferably made from a highly porous material which acts as anelectronic insulator between the aluminum/carbon composite electrodes 12and 14. The purpose of the electronic separator 18 is to assure that theopposing electrodes 12 and 14 are never in contact with one another.Contact between electrodes results in a short circuit and rapiddepletion of the charges stored in the electrodes. The porous nature ofthe electronic separator 18 allows movement of the ions in theelectrolyte 20. The preferred electronic separator 18 is a porouspolypropylene or polyethylene sheet approximately 1 mil (0.001 inches)thick. If desired, the polypropylene or polyethylene separator may beinitially soaked in the electrolyte 20 prior to inserting it between thealuminum/carbon composite electrodes 12 and 14, although suchpre-soaking is not required.

The cell holder 11 may be any known packaging means commonly used withdouble layer capacitors. Several types of packaging are describedhereinafter. In order to maximize the energy density of the double layercapacitors, it is an advantage to minimize the weight of the packagingmeans. Packaged double layer capacitors are typically expected to weighnot more than 25 percent of the unpackaged double layer capacitor.Electrical leads 28 and 29 extend from the current collector plates 22and 24 through the cell holder 11 and are adapted for connection with anelectrical circuit (not shown).

A bipolar aluminum/carbon composite electrode 30, as shown in FIG. 2A,may be utilized in combination with end portions as shown in FIGS. 2Band 2C in a corresponding series stack of such electrodes to form a highperformance bipolar double layer capacitor 40 as shown in FIG. 3. Thealuminum/carbon composite electrode 30 (FIG. 2A) comprises a polarizedaluminum/carbon composite body separated with a non-porous currentcollector plate 36. Attached to one surface 37 of the current collectorplate 36 is a charged electrode 32 for a first electrode. Attached tothe opposite surface 38 of the current collector plate 36, is anoppositely charged electrode 34. Such electrode structure may then bestacked as shown in FIG. 3, with a series stack of the bipolarcapacitors as shown in FIG. 2A being stacked between the two endportions of the stack shown in FIGS. 2B and 2C, thereby forming abipolar double layer capacitor 40. As seen in FIG. 3, if the firstelectrode 34 is a negative electrode for a first capacitor cell “A”, thesecond (or bottom) electrode of cell “A”, electrode 42, becomesoppositely charged, i.e., becomes a positive electrode. The same chargeof electrode 42 carries over to a first electrode 44 of cell “B”, i.e.,electrode 44 of cell “B” becomes positively charged relative toelectrode 34. This causes the bottom electrode 42 of cell “B” to becomeoppositely charged, i.e., negatively charged relative to electrode 44 ofcell “B”. A series stack of the high performance bipolar double layercapacitors 40 thus includes a plurality of cells (A, B, C, and D) whichare connected in series. Each cell includes a pair of aluminumimpregnated carbon composite porous electrodes. Cell “A” includeselectrodes 34 and 42 facing one another with an Ionically conductiveseparator 46 disposed between them. Cells “B” and “C” include electrodes44 and 42 facing one another with an ionically-conductive separator 46disposed between them. Cell “D” includes electrodes 44 and 32 facing oneanother with an ionically-conductive separator 46 disposed between them.A plurality of internal non-porous current collectors 48 are placedbetween each cell, having two adjoining polarized electrodes 42 and 44on each side thereof. Exterior current collecting plates 49 are placedat each end of the stack. A sufficient amount of an electrolyte 50 isintroduced within each cell such that the electrolyte 50 saturates thecomposite electrodes 32, 34, 42 or 44 and separator 46 within each cell.

The individual carbon electrode structures 32, 34, 42 and/or 44 arepreferably formed in a manner similar to the process described elsewhereherein. Each electrode structure is fabricated from a carbon clothpreform or carbon paper preform which is volume impregnated with moltenaluminum. As is explained more fully below, such impregnation serves tosignificantly reduce the electrode resistance.

More particularly, each of the electrode structures 32, 34, 42 and/or 44is fabricated from a carbon cloth preform or carbon paper preform whichis impregnated with molten aluminum. The porosity of the electrodestructures 32, 34, 42 and/or 44 should be controlled during theimpregnation process to subsequently permit a sufficient amount of theelectrolyte 50 to be introduced into the capacitor cell and penetratethe pores of the carbon fibers.

The aluminum impregnated carbon composite electrodes 32, 34, 42 and/or44 are sufficiently porous, and preferably have a sufficient aluminumimpregnant within the activated carbon fibers such that the equivalentseries resistance of each electrode when used in a 2.3-3.0 volt cell isabout 1.5 Ωcm² or less, and the capacitance of each composite electrode42 and 44 is approximately 30 F/g or greater. Such large capacitance isachievable due to the large surface area made available through the useof activated carbon fibers, and the very small separation distancebetween the capacitor layers, as explained more fully below.

The internal current collector plates 48 of each bipolar electrode arepreferably non-porous layers of aluminum foil designed to separate theelectrolyte 50 between adjacent cells. The exterior current collectingplates 49 are also non-porous such that they can be used as part of theexternal capacitor seal, if necessary.

An electronic separator 46 or porous separator is placed between theopposing electrode structures 42 and 44 within a particular internalcapacitor cell, or between opposing electrode structures 34 and 42, or44 and 32, of end capacitor cells. The electronic separator 46 ispreferably a porous polypropylene-based or polyethylene-based sheet.

Many of the attendant advantages of the present double layer capacitorresult from the preferred methods of fabricating the carbon electrodestructures, the preferred method of connecting the current collector,and the use of high performance electrolytes. Each of these aspects ofthe invention are discussed in further detail below.

As identified above, the carbon electrode structure is preferably madefrom a porous carbon fiber cloth preform or carbon fiber paper preformwhich is impregnated with molten aluminum. The preform can be fabricatedfrom any suitable activated carbon fiber material such as carbon fiberfelt or other activated carbon fiber substrates having a sufficientporosity to receive the impregnated molten aluminum and electrolyticsolution.

The aluminum, or alternatively, copper or titanium, is volumetricallyimpregnated deep into the tow of the bundles of carbon fibers within thecarbon cloth, as explained more fully below in connection with FIGS. 9A,9B, 9C, and 9D. The result of impregnating the aluminum into the tow ofthe fibrous carbon bundles is a low resistance current path between theactivated carbon elements within the electrode. However, with the lowresistance current path, the electrode structure also remainssufficiently porous so that an electrolytic solution, preferably anon-aqueous electrolytic solution, infiltrates the pores of theactivated carbon fibers.

The fabrication process of the aluminum/carbon composite electrodes ofthe double layer capacitor starts with the fabrication of a carbon fiberelectrode preform. The carbon fiber electrode preform is typicallymanufactured paper or cloth preform using high surface area carbonfibers. The preferred carbon fiber preform is carbon fiber cloth. Thecarbon fiber cloth preform is preferably a commercially available clothwhich uses woven carbon fibers also having a surface area no less than100 m²/g and typically approximately 500 to 3000 m²/g and having adiameter of approximately 8-10 μm. The carbon fiber cloth preform istypically has more structural stability than the carbon fiber paperpreform. The surface area and other dimensions of the carbon fibers,however, can be tailored to meet the requirements of the application inwhich it is used.

Impregnation of the carbon fiber cloth with molten aluminum ispreferably accomplished using a wire arc spraying or plasma sprayingtechnique, as described more fully below in connection with FIGS. 8A,8B, and 8C. Wire arc spraying molten metal onto the surface of a carbonfiber preform has previously been used in double layer capacitorconstruction as a means for forming a current collector at the surfaceof the carbon fiber preform. Thus, by definition, involves depositing athick substantially impermeable layer of metal onto the surface of thecarbon fiber cloth. However, to applicants' knowledge, wire arc sprayinghas never been done to volume impregnate the carbon fiber preform withthe sprayed metal so as to reduce the contact resistance between theactivated carbon elements, thereby forming a very low resistancecarbon/metal composite electrode made up of both the activated carbonand the impregnated metal.

The wire arc spray technique is controlled to penetrate into the carbonfiber cloth preform as described more fully below in connection withFIGS. 9A, 9B, 9C, and 9D. Control is accomplished by adjusting theelectrical current to the spray unit, the voltage, the pressure of themolten aluminum, the distance of the wire arc spray unit from the carbonfiber preform, the sweep of the wire arc spray unit, and the ambientairflow during the spraying process. Advantageously, the bulkresistivity of the carbon cloth is dramatically reduced when wire arcspraying is used to impregnate the carbon cloth with aluminum, asdescribed more fully below.

Additional details and information regarding the bipolar double layercapacitor stack shown in FIG. 3, and the electrodes used therein, may befound in patent application, Ser. No. 08/319,493, for a MULTI-ELECTRODEDOUBLE LAYER CAPACITOR HAVING SINGLE ELECTROLYTE SEAL AND ALUMINUMIMPREGNATED CARBON CLOTH ELECTRODES, by Farahmandi, et al., filed Oct.7, 1994, now U.S. Pat. No. 5,621,607, which application is incorporatedherein by reference.

Single Cell, Multi-Electrode Double Layer Capacitor

At this point, a more detailed description of a single cell,multi-electrode double layer capacitor will be presented in conjunctionwith a more detailed description of FIGS. 4A through 29. A key featureof such a double layer capacitor, as will become more apparent from thedescription that follows, is the use of multiple electrodes (or, in oneembodiment, a electrode stack or a winding assembly) connected inparallel within a capacitor package that requires only a singleelectrolyte seal. Because only one electrolyte seal is required, it isappropriate to refer to such capacitor as a “single cell” capacitorsince it is the electrolyte seal which normally defines what comprises acell. Such a single cell, multi-electrode double layer capacitorconfiguration represents the best mode for practicing the invention atthe present time. It is to be emphasized, however, that the invention isnot intended to be limited to such mode or embodiment. Rather, it iscontemplated that the invention extend to all double layer capacitorsthat use low-resistance carbon electrodes in conjunction with aluminumof the type described herein, regardless of the specific electrodeconfiguration that may eventually be used to make the capacitor, andregardless of the specific high conductivity electrolytic solution thatis employed. Such electrode configurations may include, e.g., multipleelectrodes connected in parallel in a single cell (as is described morefully herein); a pair of electrodes arranged in a spiral pattern in asingle cell; electrodes connected in series in stacked cells; or otherelectrode configurations.

Turning to FIG. 4A, a schematic representation of a two-electrode singlecell double layer capacitor 60 made in accordance with the presentinvention is illustrated. The capacitor includes two spaced apartaluminum-impregnated carbon electrodes 62 and 64 electrically separatedby a porous separator 66. The electrodes 62 and 64, as explained in moredetail below, comprise a relatively dense weave of activated carbonfibers, forming a carbon cloth, in which molten aluminum has beenimpregnated.

The electrode 62 is in contact with a current collector plate 68, whichplate 68 is in turn connected to a first electrical terminal 70 of thecapacitor 60. Similarly, the electrode 64 is in contact with anothercurrent collector plate 72, which plate 72 is connected to a secondelectrical terminal 74 of the capacitor 60. The region between theelectrodes 62 and 64, as well as all of the available spaces and voidswithin the electrodes 62 and 64, are filled with a highly conductivenon-aqueous electrolytic solution 76. The ions of the electrolyticsolution 76 are free to pass through pores or holes 65 of the separator66; yet the separator 66 prevents the electrode 62 from physicallycontacting, and hence electrically shorting with, the electrode 64. Apreferred separator, for example, is polypropylene-based. Polypropyleneincludes pore openings having dimensions on the order of 0.04 by 0.12μm. This size pore prevents the fibers of the carbon cloth, which have adiameter on the order of 8-10 μm, from poking through the pores. Anothersuitable separator material is comprised of polyethylene. Polyethylenegenerally has pore sizes on the order of 0.1 μm diameter or less,thereby also preventing carbon fibers having a minimum diameter of 8 μmfrom poking therethrough.

In operation, when an electrical potential is applied across theterminals 70 and 74, and hence across the series-connected electrodes 62and 64, a polarized liquid layer forms at each electrode immersed in theelectrolyte. It is these polarized liquid layers which storeelectrostatic energy and function as the double layer capacitor—i.e.,that function as two capacitors in series. More particularly, asconceptually depicted in FIG. 4A by the “+” and “−” symbols(representing the electrical charge at the electrode-electrolyteinterface of each electrode that is immersed in the electrolyte), when avoltage is applied across the electrodes, e.g., when electrode 62 ischarged positive relative to electrode 64, a double layer is formed(symbolically depicted by the two “+/−” layers shown in FIG. 4A) by thepolarization of the electrolyte ions due to charge separation under theapplied electric field and also due to the dipole orientation andalignment of electrolyte molecules over the entire surface of theelectrodes. This polarization stores energy in the capacitor accordingto the following relationships:

 C=k _(e) A/d  (1)

and

E=CV ²/2  (2)

where C is the capacitance, k_(e) is the effective dielectric constantof the double layer, d is the separation distance between the layers, Ais the surface area of the electrodes that is immersed in theelectrolytic solution, V is the voltage applied across the electrodes,and E is the energy stored in the capacitor.

In a double layer capacitor, the separation distance d is so small thatit is measured in angstroms, while the surface area A, i.e., the surfacearea “A” per gram of electrode material, may be very large. Hence, ascan be seen from Eq. (1), when d is very small, and A is very large, thecapacitance will be very large.

The surface area “A” is large because of the make-up of the electrodes,each electrode comprising a weave of activated carbon fiber bundles toform a carbon cloth. The activated carbon fibers do not have a smoothsurface, but are pitted with numerous holes and pores 80, as suggestedby FIG. 4B. That is, FIG. 4B conceptually illustrates a small section ofan activated carbon fiber 78 having numerous pits or holes 80 therein.The fiber 78, as previously indicated, typically has a diameter on theorder of 8-10 μm; while the pits or holes of the activated carbon fiberhave a typical size of about 40 angstroms. The fiber 78 is immersed inan electrolytic solution 76. Each pit or hole 80 significantly increasesthe surface area of the fiber that is exposed to the electrolyticsolution 76. Because there are a large number of fibers 80 in eachbundle, and because there are several bundles within the weave that formthe carbon cloth, the result is a three-dimensional electrode structurewhich allows the electrolyte to penetrate into the weave of the fibersand contact all, or most all, of the surface area of the fibers, therebydramatically increasing the surface area “A” of the electrode over whichthe double layer of charged molecules is formed.

By way of example, a suitable carbon cloth that may be used to make theelectrodes of the present invention is commercially available. Thediameter of the carbon fibers of such cloth, such as the fiber 78 shownin FIG. 4B, is on the order of 8 microns (8×10⁻⁶ m); whereas the overallthickness of the carbon cloth is about 0.53 millimeters (mm). Theaverage diameter of the pores in the activated carbon fibers is some 44angstroms, and the pore/void volume is about 1.2 ml/g. It should benoted that the pore/void volume results from three different types ofvoids or pores in the cloth: (1) the pores or pits in the individualactivated carbon fibers (such as the pores 80 shown in FIG. 4B thatcover most of the surface area of the activated carbon fibers); (2) thespace between the fibers that form a carbon bundle (which space, forpurposes of the present invention, when viewed in a cross section, as inFIG. 9B, is referred to as the “tow” of the fiber bundle); and (3) thevoids between the fiber bundles that are woven to form the cloth. Suchpore volume results in an overall surface area of the carbon cloth ofabout 2500 m²/g. Because of the pore/void volume of the cloth, the clothis somewhat spongy, and therefore compressible. The density of the clothis typically about 0.26 g/cm³, resulting in an theoretical effectivearea/unit-volume (i.e. void volume) of about 650 m²/cm³. With such anarea/unit-volume, it is thus possible, see Eq. (1) to achievecapacitances on the order of 6 F/cm³.

Achieving a high capacitance, however, is only part of the invention. Ifsuch high capacitance is to be of practical use, it must be able tostore and discharge energy in a relatively quick time period. Thecharge/discharge time of a capacitor, as discussed more fully below, isgoverned by the internal resistance of the capacitor. The lower theinternal resistance, the shorter the charge/discharge time.

The internal resistance of the basic double layer capacitor 60 depictedin FIG. 4A is made up of several components, as illustrated in theequivalent circuit diagram of the capacitor 60 shown in FIG. 5. As seenin FIG. 5, the internal resistance of the double layer capacitor 60includes a contact resistance, R_(c), which represents all of theresistance in the current path between the capacitor terminal 70 up tothe electrode 62 (represented in FIG. 5 as the upper plate of capacitorC1), or all of the resistance in the current path between the capacitorterminal 74 and the electrode 64 (represented in FIG. 5 as the lowerplate of capacitor C2).

As further seen in FIG. 5, the internal resistance of the capacitor 60also includes an electrode resistance, R_(EL), which represents theresistance within the electrode 62 (or within the electrode 64) betweenthe surface of the carbon cloth used to make the electrode and all ofthe individual activated carbon fibers used within the carbon cloth,i.e., R_(EL) represents the internal contact resistance between thecarbon fibers within the electrode. Additionally, an electrolyticsolution resistance, R_(ES), exits relative to the electrolytic solution76; and a separator resistance, R_(SEP), exists relative to the porousseparator 66.

Any energy stored within the double layer capacitor 60 must enter orexit the capacitor by way of an electrical current that flows throughR_(c), R_(EL), R_(ES), and R_(SEP). Thus, it is seen that in order forpractical charge/discharge times to be achieved, the values of R_(c),R_(E), R_(ES), and R_(SEP), which in combination with the capacitance Cor C₁+C₂ define the time constant τ_(c) of the capacitor, must be keptas low as possible.

The resistance of the separator, R_(SEP), is a function of the porosityand thickness of the separator. A preferred separator material iscomprised of polypropylene having a thickness of about 0.001 inches(0.025 mm). An alternative separator material is comprised ofpolyethylene, also having a thickness of about 0.001 inches (0.025 mm).The polypropylene separator inherently has a smaller pore size with a20-40% porosity. The polyethylene separator has a larger pore size witha 60-80% porosity yet has a more tortuose or twisted path than thepolypropylene separator in which the electrolyte ions may flow. Thepolypropylene separator has a sheet structure while the polyethyleneseparator has a more lamellar structure.

The resistance of the electrolytic solution is determined by theconductivity of the particular electrolytic solution that is used. Inselecting the type of electrolytic solution to use, several tradeoffsmust be considered. Aqueous electrolytic solutions generally have ahigher conductivity than do non-aqueous solutions (e.g., by a factor of10 to 100). However, aqueous solutions limit the working voltage of thecapacitor cell to around 0.5 to 1.0 volt. Because the energy stored inthe cell is a function of the square of the voltage, see Eq. (2) above,high energy applications are probably better served using a non-aqueouselectrolyte, which permits cell voltages on the order of 2.0 to 3.0volts. As previously indicated, the preferred electrolyte for use withthe double layer capacitor described herein is made from a mixture ofacetonitrile (CH₃CN) and a suitable salt, which mixture exhibits aconductivity on the order of 50 ohm⁻¹/cm. It is to be emphasized,however, that the invention herein described contemplates the use ofalternate electrolytic solutions, particularly non-aqueous (or organic)electrolytic solutions, other than the solution made from acetonitriledescribed above. For example, several alternative electrolytic solutionsare disclosed in the previously cited U.S. patent application Ser. No.08/319,493, for MULTI-ELECTRODE DOUBLE LAYER CAPACITOR HAVING SINGLEELECTROLYTE SEAL AND ALUMINUM-IMPREGNATED CARBON CLOTH ELECTRODES, byFarahmandi, et al., filed Oct. 7, 1994, now U.S. Pat. No. 5,621,607.

The contact resistance R_(c) in combination with the electroderesistance R_(EL) represent a significant source of internal resistanceof the capacitor 60. A high electrode resistance has heretofore been amajor stumbling block in the development of commercially viable, highenergy density, double layer capacitors. A key feature of the presentinvention is to provide a double layer capacitor having a very lowelectrode resistance in combination with a high energy density. A majorobjective of the present invention is to reduce R_(c)+R_(EL) to a valuethat is small in comparison to R_(SEP). To that end, much of thediscussion that follows focuses on manufacturing and assembly techniquesthat reduce the electrode resistance, R_(EL), as well as the contactresistance, R_(c).

Alternatively, C₁ and C₂ in FIG. 5 may each represent the totalcapacitance of multiple electrodes in parallel, such that C₁ is theequivalent capacitance of an electrode stack connected in parallel.

To further illustrate the significant role that the electrode resistanceR_(EL) plays in the operation of the double layer capacitor 60 of thepresent embodiment, reference is next made to FIG. 6A. FIG. 6A shows anequivalent circuit diagram of an aluminum impregnated carbon clothelectrode double layer capacitor 60. Unlike the representation in FIG.5, the electrode resistance R_(EL) is represented as a series ofseparate resistances R_(EL1), R_(EL2), R_(ELn), signifying increasingresistance as a function of distance in the activated carbon fiber(electrically speaking) through which a particular portion of thecurrent travels before passing into the electrolyte (as ionic current).

Typically, current entering and exiting activated carbon fibers near thecurrent collector, sees a relatively lower electrode resistance thandoes current that travels through activated carbon fibers through theentire thickness of the carbon fiber cloth before passing into theelectrolyte.

At the same time, current that passes into the electrolyte near thecurrent collector foil (after having traveled relatively little distancethrough the activated carbon fibers) has a greater path distance throughthe electrolyte solution and thus a greater electrolyte solutionresistance R_(ES), than does current that passes into the electrolytesolution after having traveled through the entire thickness of thecarbon cloth, and thus has a lesser electrolyte solution resistanceR_(ES). FIG. 6A depicts schematically near the inverse relationshipbetween R_(EL) and R_(ES) through a series/parallel circuit having a“ladder” structure on which the individual capacitance functions of eachunit of surface area are the “rungs” of the ladder and a series ofindividual electrode resistances form one “leg” of the ladder, and aseries of individual electrolyte solution resistances form another “leg”of the ladder. The contact resistance is coupled to one end of the oneleg, and the separator resistance is coupled to another end of the otherleg, such that current traveling through each individual capacitance has“seen” at least one of the electrode resistances and at least one of theelectrolyte solution resistances, with the number of, i.e., the amountof resistance of, the electrode resistances being inversely proportionalto the number of, i.e., the resistance of, the electrolyte solutionresistances.

FIG. 6A further illustrates a first portion of current taking path “A”entering and exiting the carbon fiber cloth relatively near to thecurrent collector foil/carbon fiber cloth interface, and traveling arelatively greater distance through the electrolyte solution, a secondportion of current taking path “B” entering the carbon fiber cloth atthe current collector foil/carbon fiber cloth interface and exiting atan intermediate position, with an intermediate distance of travelthrough the electrolytic solution, and a third portion of current takingpath “C” entering the carbon fiber cloth at the current collectorfoil/carbon fiber cloth after having passed through the entire thicknessof the carbon fiber cloth, with a relatively shorter distance of travelthrough the electrolyte solution.

Understanding of these sources of resistance by the inventors remainssignificant to their success at reducing the resistances to a level thatpermits the making of a commercially viable, practical, high-voltage,low internal resistance, small size, long life, double layer capacitor.

Advantageously, in the present embodiment, total resistance seen by theentire amount of current passing through the double layer capacitor isno more than 900 μΩ.

Referring next to FIG. 6B, a diagram is shown that conceptually showsthe alternate paths ions may take as the current flows through a singleelectrode to illustrate resistance and capacitance at various points inthe electrode in accordance with one embodiment of the presentinvention. Shown are a single electrode 82 having been impregnated withmetal, a single current collector 84 and a single separator 86. Alsoshown are carbon fiber bundles 88 within the carbon fiber cloth 82

A charge may take path “D” through the electrode 82 experiencingR_(SEP), then enter the electrode 82 until it enters a carbon fiberbundle 88. Then, the charge travels axially through the carbon fiberbundle 88 to the current collector 84. The charge experiences resistancefrom the electrode, R_(EL), and resistance from the electrolytesolution, R_(ES). The current then flows through the collector foil 84,experiencing R_(c). As shown by the different paths “D”, “E”, and “F”,the amount of electrode and solution resistance varies for each chargeand the path it takes. A charge taking path “D” experiences moresolution resistance (R_(ES)) and less electrode resistance (R_(EL)) thana charge in path “F”, for example. Each path generates a separatecapacitance as well. The effective capacitance is the sum of theseparate capacitances C₁ through C_(N).

A simplified circuit that illustrates the use of a capacitor as a powersource to deliver energy to a load, R_(L), is shown in FIG. 7. In FIG.7, all of the capacitor resistances shown in FIG. 5, including thecontact resistance 2×R_(c) associated with both terminals, the aluminumimpregnated carbon cloth electrode resistance 2×R_(EL) and furtherincluding the electrolytic solution resistance 2×R_(ES) and theseparator resistance R_(SEP) (if not sufficiently low to be neglected),are included in the capacitor resistance R_(z).

The total resistance RT of the power delivery circuit in FIG. 7 is

R _(T) =R _(z) +R _(L).  (3)

The total time constant τ of the power delivery circuit is thus:

τ=R _(T) C,  (4)

whereas the time constant τ_(c) of just the capacitor is

τ_(c) =R _(z) C.  (5)

The voltage developed across the load V_(L) is

V _(L) =V _(O)(R _(L) /R _(T))=V _(O)(1−R _(C) /R _(T))  (6)

and the power delivered to the load is

P=IV _(L) =IV _(O)(1−R _(C) /R _(T))=IV _(O)(1−CR _(C) /CR _(T))  (7)

or

P=IV _(O)(1−τ_(c)τ).  (8)

The expression (1−τ_(c)/τ) represents the efficiency rating ε of thepower delivery circuit, i.e.,

ε=(1−τ_(c)/τ).  (9)

The degree to which the power source (in this case the capacitor Ccharged to a voltage V_(O)) is able to efficiently deliver power to theload, R_(L) is thus highly dependent upon the characteristic RC timeconstant of the capacitor τ_(c). The characteristic RC time constant ofthe capacitor, in turn, is directly related to the resistance of thecapacitor, R_(z). For an efficient power delivery circuit to be achievedusing the double layer capacitor C, it is thus apparent that theresistance of the capacitor, R_(z) , must be minimized so that a lowtime constant of the capacitor T_(c) can be realized. In one embodiment,R_(z) is less than 900 μΩ.

Advantageously, the present invention provides a multi-electrode doublelayer capacitor of the type represented in the equivalent circuit ofFIG. 6 that, when configured substantially as described below inconnection with FIGS. 9A through 22B, has performance specifications asset forth in Table 1. Such configuration (i.e., the configuration shownin FIGS. 21A and 21B) may be referred to herein as the PC 2500 or“prismatic” design double layer capacitor. Significantly, a capacitoroperating in accordance with the specifications shown in Table 1exhibits a time constant τ_(c) of about 2 seconds. The energy densityachieved is in the range of 2.9-3.5 W-hr/kg, and the power rating isover 1000 W/kg (at 400 A). Such performance in a single cell doublelayer capacitor, to applicants' knowledge, has never been achievedbefore.

TABLE 1 Performance Specifications of PC 2500 Parameter Value UnitsCapacitance 2,500 Farad Tolerance +5 % Rated Voltage 2.3 Volts Max SurgeVoltage 2.7 Volts Rated Energy 6,600 Joules ESR*, room temp 900 μΩ ESR*,high temp 2.7E+3 μΩ ESR*, low temp 900 μΩ Case Style Elongated can withlid Electrical Connection Two terminals having opposite polarity CaseDimensions 2.375 × 2.375 × 6.1 inches 60.1 × 60.1 × 155 mm Approx.Weight 1.5 lbs 0.68 kg Electrolyte: Organic Impregnant (solvent + salt)solvent: acetonitrile (CH₃CN) salt: tetraethylammonium tetraflouraborate(CH₃CH₂) ₄N⁺BF₄ ⁻ Ratio salt/solvent: 303.8 g/liter (*ESR = ElectrodeSeries Resistance)

Prismatic Design, PC 2500

Turning next to FIGS. 8A through 21B, the basic technique used in makinga double layer capacitor in accordance with the “prismatic” designembodiment of the present invention will be described. FIGS. 22A and 22Bare a flow chart that illustrates the main steps in such process; whileFIGS. 8A through 21B illustrate individual steps of the process. Hence,in the description of the assembly and fabrication process that follows,reference will be made to specific blocks or boxes of the flow chart ofFIGS. 22A and 22B to identify particular steps, at the same time thatreference is made to respective ones of FIGS. 8A through 21B toillustrate the step being carried out.

Referring to FIG. 22A, a flowchart that illustrates the method of makingand assembling a wrapped electrode stack or winding assembly of anembodiment to be used in a “prismatic” design double layer capacitor.

With reference first to block 2204 of FIG. 22A, and with reference alsoto FIGS. 8A, 8B, and 8C an initial step to be carried out in making acapacitor 60 (FIG. 5) in accordance with the present invention is toplasma spray or wire arc spray a suitable carbon cloth 92 (FIG. 8A) withmolten aluminum spray 94 so that the aluminum is impregnated deep intothe tow of the fibers of the carbon cloth 92. The carbon cloth 92 to besprayed is preferably a commercially-available carbon cloths as known inthe art. As seen in FIGS. 8A and 8C, the carbon cloth 92 is typicallyobtained in a roll 96. The roll is typically about 36 inches wide. Alength of carbon cloth 92 is extracted from the roll 96 and held in asuitable frame 98. The frame includes a backup mesh 93. The frame ispositioned in front of a spray nozzle 100. The frame 98 exposes a“window” of the cloth having approximate dimensions of 2.31 inches by34.25 inches, to the plasma spray 94. The wire arc spray nozzle iscontrolled by an X-Y controller 102 to provide a desired spray patternon the carbon cloth.

The molten aluminum spray 94 is formed by feeding two aluminum wires 104and 106 from respective rolls of aluminum wire into the nozzle 100 at acontrolled rate. The wires 104 and 106 are not limited to aluminum wiresand may comprise another suitable metal, such as copper or titanium. Thetips of the wires are held within the nozzle a specified distance apart.A source of electrical power 108 causes an electrical current to flowthrough the wires and arc across the tips of the wires. The electricalarcing causes the aluminum to melt. As the aluminum melts, it is carriedout of the nozzle 100 in a stream by compressed air, provided by aircompressor 110. As the aluminum is spent and carried away in the moltenaluminum stream 94, additional aluminum wire 104, 106 is metered intothe nozzle 100 to maintain the desired gap for the electrical arc. Inthis manner, a source of aluminum is continually metered into the nozzleso that a constant stream of molten aluminum can be directed at thecarbon cloth.

The molten stream of aluminum is sprayed onto and into the carbon cloth92 following an over-up-and-back spray pattern as shown in FIG. 8B. Thebackup mesh 93, which has mesh openings on the order of 0.0036 in²,allows the molten aluminum flow to continue through the cloth tooptimize the volume impregnation with aluminum. The wires 104 and 106are not limited to aluminum wires and may comprise another suitablemetal, such as copper or titanium. The aluminum wires 104, 106 arepreferably 99.5% pure aluminum having a diameter of about {fraction(1/16)}th of an inch.

In operation, all of the operative equipment shown in FIG. 8A, e.g., thenozzle 100, X-Y controller 102, frame 98, and wires 104, 106 are placedin a wire arc spray chamber (to confine the aluminum dust). The air inthe compressor is dried. An exhaust fan 112 maintains a constant flow ofair through the chamber in the direction away from the nozzle 100. Thecloth 92 is manually clamped in the frame 98, and a single spray patternis performed. Only one side of the cloth is sprayed. Once sprayed, thecloth is released from the frame. A new length of unsprayed carbon cloth92 is then indexed in the frame, as needed, for the next strip of carboncloth to be sprayed.

Referring to FIG. 8C, an alternate system is shown for jet sprayingmetal, preferably aluminum, into a carbon cloth. The system of FIG. 8Ccontains similar elements as FIG. 8A. Shown are an air compressor 110having a primary line 114 and a secondary line 116, a source ofelectrical power 108, an x-y controller 102, and aluminum wires 106 and104, all of which connect to an arc spray nozzle 100. The arc spraynozzle 100 has a jet spray nozzle 101 attached. Also shown are thecarbon cloth 92, backup mesh 93, exhaust fan 112, and molten metal spray94.

The operation is similar to the operation of the arc spraying system ofFIG. 8A, except that the jet spraying is done downward onto the carboncloth 92 which is resting on a backup mesh 93. There is no need to use aframe since the carbon cloth 92 does not need to be held in place.Alternatively, a similar frame or guiding mechanism could be used.Additionally, the arc spray nozzle 100 has an additional jet spraynozzle 101 attached. This jet spray nozzle 101 is actually threeseparate spray nozzles directed toward the carbon cloth 92. The aircompressor 110 sends compressed air into the arc spray nozzle 100through the primary line 114 at about 50 psi similar to FIG. 8A;however, another line, the secondary line 116, carries compressed airinto the jet spray nozzle 101 at about 40 psi. This secondary line 116further boosts the strength of the molten metal spray 94 against thecarbon cloth 92. Using the embodiment in FIG. 8C, less Aluminum isactually sprayed in less time while achieving a more effectiveimpregnation of the carbon cloth 92 than in the wire arc spray techniquedescribed in FIG. 8A. The impregnation depth using the system of FIG. 8Cis about the same as the impregnation depth using the system of 8A andis typically about ¼ through the entire carbon cloth 92 or about ⅔ to ¾through just the top carbon fiber bundle of the carbon cloth 92.Additionally, the jet sprayer of FIG. 8C allows less molten metal (i.e.aluminum) to build up on the surface of the carbon cloth 92 than thewire arc sprayer of FIG. 8A.

The operating parameters used during the arc spray process are asfollows: The electrical current used to melt the aluminum is 80-90amperes at an arc voltage of about 31 V. The compressed air ismaintained at a pressure of approximately 50-60 psi in FIG. 8A, and 60and 40 psi for the primary line 114 and the secondary line 116,respectively, in FIG. 8C. The distance between the tip of the nozzle 100and the cloth is 20 inches in FIG. 8A and between 4.5 to 6 inches inFIG. 8C. The complete spray pattern is traversed at a constant rate in atime period of about 45 seconds for FIG. 8A and about 1 second for FIG.8C. The nozzles 100 and 101 are adjusted so that the stream of moltenaluminum 94 covers the carbon cloth 92 as uniformly as possible withminimum overlap.

Once the arc spraying process has been completed, a layer of aluminum ispresent on the front side of the carbon cloth 92, and there should be aslight visual pattern of the backup mesh 93 visible on the back side ofthe carbon cloth. Such pattern provides visual verification that atleast some aluminum has penetrated all the way through the carbon clothto optimize volume impregnation during the arc spraying process. Thelayer of aluminum using the jet sprayer of FIG. 8C (see FIG. 9C) is muchthinner than the layer of aluminum formed using the wire arc sprayer ofFIG. 8A (see FIG. 9A).

All of the equipment referenced in FIGS. 8A-8C is conventional. Thedetails and manner of operating such equipment are known to those ofskill in the art.

The purpose of spraying the carbon cloth 92 with the aluminum is toreduce the transverse resistance through the carbon cloth 92. Measureddata of the electrode series resistance (ESR), taken before and afterwire arc spraying using the wire arc sprayer of FIG. 8A and with variousamounts of aluminum is summarized in Table 2.

TABLE 2 Aluminum Capacitance ESR of Capacitor Density (mg/cm³) (F/g)(Ω-cm²) 0 (unsprayed) 115 52.0 157 >130 1.509 209 >140 1.299 250 1471.26 410 144 1.08 509 >130 1.308

The data in TABLE 2 was taken using electrodes that were 2500 m²/g cutto 5.1 cm in diameter and that contained approximately 0.2 g of carbon.The carbon density in the unsprayed cloth was 0.26 g/cm³.

As seen from the data in TABLE 2, the resistance of a carbon cloth 92that has been wire arc sprayed with aluminum reduces the resistance ofthe carbon cloth by up to a factor of 50. Such a dramatic reduction inresistance, which is caused by a decrease in the volumetric resistivityof the electrode structure, directly influences the electroderesistance, R_(EL), and thus significantly improves the ability of thecapacitor to exhibit a low time constant.

As further seen from the data in TABLE 2, reducing the resistance of theelectrode through impregnation of aluminum is a process that must beoptimized in order to produce the lowest resistance for a desired amountof aluminum. Too little aluminum and the resistance remains too high.Too much aluminum, and the weight of the electrode is increasedsufficiently to degrade the energy density. Too much aluminum alsoblocks the electrolyte from penetrating into the carbon weave so as tocontact all of the surface area of the fibers, thereby effectivelydecreasing the available surface area.

It is significant that the aluminum spray 94 which is directed at thecarbon cloth 92 (FIG. 8A) does much more than just coat the surface ofthe carbon cloth 92 with aluminum. While the aluminum certainly doescoat the surface, it also penetrates into the cloth or reaches into thevoids (or interstices) in between the carbon fiber bundles of the carbonfiber cloth and; thus, impregnates the cloth with aluminum. Thesignificance of impregnating the cloth with aluminum is best illustratedwith reference to FIGS. 9A, 9B, 9C, and 9D.

FIG. 9A shows a schematic representation of a side sectional view of thecarbon cloth 92. As seen in FIG. 9A, the carbon cloth 92 is made up of aplurality of fiber bundles 120 that are woven to form the carbon cloth92. For simplicity, only four such fiber bundles 120 are shown in FIG.9A. Each fiber bundle 120 is made up of many carbon fibers 122, as seenbest in FIG. 9B, which conceptually illustrates a cross-sectional viewof an individual fiber bundle 120.

The axial resistance of the individual carbon fibers 122 is very low,but the transverse resistance through a carbon bundle 120 is relativelyhigh. It is this transverse resistance, i.e., the resistance from Point“A” on one side of the cloth 92 to Point “B” on the other side of thecloth, which must be lowered in order to reduce the electrode resistanceR_(EL). Wire arc spraying the carbon cloth 92 with an aluminum spray 94advantageously causes the aluminum to flow into the tow 126 of thebundle 120, as shown in FIG. 9B. Such penetration, or impregnation, intothe tow of the fiber bundle 120 thereby reduces the contact resistancebetween the individual fibers 122. Thus, the sprayed aluminum fills someof the voids in the carbon fiber cloth 92. The aluminum reaches into thetow, or in between the interstices of the carbon fiber bundles 121. Theresulting low transverse contact resistance together with the intrinsiclow axial resistance of the fibers then permits a very low resistancepath to be made completely through the width of the cloth 92, i.e.,provides a very low transverse resistance through the electrodestructure. The transverse resistance of the current flow from Point “A”to Point “B” is also influenced by the weave type, fiber tow size, andthe twist of the fiber tow. A more efficient and repetitive path forcarrying current from Point “A” to Point “B” can be created withoptimization of the above parameters.

Furthermore, the impregnation process does not significantly effect theporosity of the carbon cloth 92. The porosity is maintained on amicroscopic level such that sufficient electrolytic solution may beenter the pores of the carbon fiber bundles. Thus, even though the metalimpregnant takes up some of the void volume of the carbon cloth, it isnot small enough to interfere with the porosity of the carbon cloth 92;and, therefore the porosity of the carbon cloth 92 is maintained duringthe impregnation process. The resulting area/unit-volume of the carboncloth having been sprayed or the void volume of the carbon cloth 92having been sprayed is about 600 m²/cm³. On the other hand, if too muchmetal is impregnated into the carbon cloth, the metal may act as abarrier to the electrolytic solution being able to penetrate into thecarbon cloth itself.

When the aluminum spray 94 strikes the cloth 92, it not only impregnatesthe tow 122 of the fiber bundle 120 with aluminum, as described above,but it also forms a layer 124 of aluminum on the sprayed surface of thecarbon cloth. The layer 124 of aluminum contours to the shape of thesurface of the carbon cloth 92. FIG. 9A illustrates the resulting layer124 formed using the wire arc sprayer of FIG. 8A. The jet sprayer ofFIG. 8C results in a much thinner layer 124 of aluminum (see FIG. 9C).Note that the layer 124 is uneven and is not intended to be used as acurrent collector. In addition, some of the aluminum also fills some ofthe voids 128 between the fiber bundles. The aluminum layer 124 helps tomake good (low resistance) electrical contact with the foil currentcollectors 68 and 72 (FIG. 4A). That is, the aluminum layer 124 servesto lower the contact resistance, R_(c). The presence of aluminum in thevoids 128 between the fiber bundles adds weight to the electrode andshould thus be minimized after achieving adequate volumetric resistivityand a low characteristic RC time constant.

Referring to FIG. 9C, a representation of a side sectional view of thecarbon cloth 92 made out of triple twisted carbon fiber bundles 121 isshown. A cross-section of one of the triple-twisted carbon fiber bundleis shown and further described in FIG. 9D. The carbon cloth 92 isentirely weaved from the triple twisted carbon fiber bundles 121.Further illustrated is the layer 124 of aluminum formed using the jetsprayer of FIG. 8C. The layer 124 (in FIG. 9C) formed using the jetspray technique is thinner than the layer 124 (in FIG. 9A) formed withthe wire arc spray technique. Typically, the layer 124 formed with thejet spray technique is not more than ¼ of the thickness of a singlecarbon fiber bundle 121 (or carbon fiber bundle 120 if using the carboncloth in FIG. 9A).

Referring to FIG. 9D, conceptually shown is a cross sectional view of atriple twisted carbon fiber bundle 121 in the embodiment of the carboncloth shown in FIG. 9C. Three carbon fiber bundles 123 have individualfibers and the tow of each carbon fiber bundle is shown, as well as theideal impregnation depth of metal into the triple twisted carbon fiberbundle 121.

The three carbon fiber bundles 123 are twisted together to form a tripletwisted carbon fiber bundle 121, which is about the same size as thecarbon fiber bundle 120 of FIGS. 9A and 9B. The carbon cloth of FIG. 9Cwill be woven out of many triple twisted carbon fiber bundles 123. Thetwisting rotates the individual carbon fibers as they extend radiallythrough the length of the fiber bundle 123; thus, less aluminum needs tobe impregnated to reach all of the individual fibers of the tripletwisted fiber bundle 123. This decreases the amount of carbon to carboncontacts within the fiber bundle 123 and; therefore, lowers thetransverse resistance of the carbon cloth 92 using triple twisted fiberbundles even further than with the single fiber bundles 120 in FIG. 9B.The twisting force displaces the shape of the carbon fibers, especiallyat the edge of each carbon fiber bundle 123, where the carbon fiberbundles 123 begin to fray slightly; thus, allowing more aluminum 94 tobe impregnated within the tow 126. Thus, the flow of current from thedirection of Point “B” to Point “A” in FIG. 9A is improved with thetriple twisted fiber bundle shown in FIG. 9C. By varying the twist andthe tow size of the carbon fiber bundles 121, the transverse resistancecan be lowered; thus, optimizing the transverse current flow in thecarbon cloth 92.

The ideal impregnation depth of the aluminum into the tow 126 of thecarbon fiber bundles 120 or the triple twisted carbon fiber bundles 121has not yet been quantified. It is believed, however, that theimpregnation pattern, when viewed in cross-section, is similar to thatillustrated in FIG. 9B and 9D, filling about ⅔ to ¾ of the available towvolume at the point where the bundle is exposed at the surface of thecloth using either the wire arc spraying technique in FIGS. 8A and 8B orthe jet spray technique in FIG. 8C. In other words, the impregnationdepth is approximately {fraction (1/4 )} through the entire carbon cloth92.

The weight of aluminum retained on or in the carbon cloth is maintainedat between about 20-30%, e.g., 25%, of the total weight of the carboncloth plus aluminum, or about 15% of the total weight, including theelectrolyte for the jet spray technique of FIG. 8C. In contrast, thewire arc spray technique of FIG. 8A results in a much thicker layer 124;thus, the weight of aluminum retained on the carbon cloth is about 50%of the total weight of the carbon cloth plus aluminum, including theelectrolyte.

Returning to FIG. 22A, it is seen that after the carbon cloth has beensprayed and impregnated with aluminum (block 2204), the impregnated rollof carbon cloth is sliced into strips having a width for the desiredcapacitor assembly, typically greater than 2 by 10 inches; thus, formingcarbon cloth strips (block 2206). For the “prismatic design”, thedimension is about 2.2 by 10.8 inches. Alternatively, in an automatedassembly process, the impregnated roll of carbon cloth is sliced intosmaller rolls having a width of the respective part, in the case of the“prismatic” design, about 10.8 inches. The smaller rolls are thenunrolled and cut into the desired size strip needed for assembly of theelectrode.

Referring again to FIG. 22A, in a parallel path to preparing theimpregnated carbon cloth strips, the current collector foils are alsoprepared, indicated in step 2202. A first step in preparing the currentcollector foils is to precut a sheet of aluminum foil into coupons(block 2202). Next, the coupons of aluminum foil is die cut to the exactdimensions necessary (block 2210). FIG. 1OA (see below) shows thedesired shape of the current collector foil used in the “prismatic”design embodiment. In order to die cut, it is necessary to place sheetsof a stiff paper, such as white butcher paper, having the same size asthe aluminum foil coupons in between the coupons. The paper addsrigidity to the foil during the die cut and also prevents the couponsfrom cold fusing together.

Referring to FIG. 22A, after the metal impregnated carbon clothelectrodes and aluminum current collector foils are formed, theelectrodes are formed, shown by block 2212. The steps of forming theelectrodes are illustrated in FIGS. 10A-10E.

Thus, Referring to FIGS. 10A through 10E, an illustration is shown formaking an electrode to be used in a stack of electrodes or windingassembly in “prismatic” design double layer capacitor embodiment of thepresent invention. Referring to FIG. 10A, a current collector foil 1000having a tab portion 1004 and a paddle portion 1002 is shown.

The current collector foil 1000 is constructed from a sheet of aluminumfoil with a thickness of approximately 0.002 inches. The foil is cut toa shape substantially as shown in FIG. 10A. The tab portion 1004 doesnot have to flush the edge of the paddle portion 1002, so long as thetab portion 1004 is offset from a central axis 1006 of the paddleportion 1002. Since the tab portion 1004 is offset from the central axis1006 of the paddle portion 1002 as shown, multiple current collectorfoils can be stacked on top of each other so that the tab portions face,generally the same direction but do not touch (see FIG. 13). The tabportion 1004 and the paddle portion 1002, thus, comprise a currentcollector foil 1000 (sometimes referred to as the current collectorplate). The current collector foil 1000 is about ten inches long. Thepaddle portion 1002 is about 5.2 inches long, and the tab portion 1004is about 4 inches long. The paddle portion 1002 has a width of about 2inches, and the tab portion 1004 has a width of about 0.75 inch.

Referring to FIGS. 10B, 10C, 10D, and 10E, an illustration of how tomake an electrode with an impregnated carbon cloth and a currentcollector foil 1000 of FIG. 10A is shown. The elements illustrated arethe current collector foil 1000 having a paddle portion 1002 and a tabportion 1004, and the metal impregnated carbon cloth 1008 having acentral fold line 1010 and an arc sprayed surface 1012.

FIG. 10B shows a top view of a strip of metal impregnated carbon cloth1008 with the current collector foil 1000 of FIG. 10A positioned thereonis shown. The metal impregnated carbon cloth 1008 is formed by arcspraying metal into the carbon cloth strip as discussed with referenceto FIGS. 9A-9D. The metal (preferably aluminum) impregnated carbon cloth1008 is shown having the arc sprayed surface 1012 facing up or towardthe current collector foil. The current collector foil 1000 ispositioned such that the carbon cloth overlaps the sides of the paddleportion 1002 by a small amount. The arc sprayed carbon cloth 1008 isthen folded over a central line 1010 such that the first end of thecloth and the second end of the cloth meet, shown as a side view inFIGS. 10C and 10D. Thus, the arc sprayed surface 1012 of the carboncloth 1008 contacts the current collector foil 1000. The electrode 1114comprises the current collector foil 1000 and the aluminum sprayedcarbon cloth 1008 contacting the paddle portion 1002 of the currentcollector foil 1000. FIG. 10D is a side view of the electrode 1014 withthe impregnated carbon cloth 1008 folded against the paddle portion 1002so that the tab portion 1004 extends freely from the electrode 1014. Atop view of the electrode 1014 is shown with reference to FIG. 10E.Again the tab portion 1004 is offset from the central axis 1006 of thepaddle portion 1002 of the current collector foil 1000. Alternatively,the impregnated carbon cloth strips could be cut along the central line1010, such that the current collector has one impregnated carbon clothstrip 1008 on one side and another impregnated carbon cloth strip 1008on the other side, instead of one piece of cloth folded around thecurrent collector foil.

Once the electrode 1014 is formed (Block 2212 of FIG. 22A), theelectrode 1014 is pressed with a mechanical press. The electrode 1014may be pressed at 1800 psi, for example. The impregnated carbon cloth1008 is compressible or somewhat spongy, so application of this pressureserves to compress somewhat the weave of the fiber bundles so as to makethe impregnated carbon cloth 1008 thinner by about 15-20%. Thisreduction in the thickness of the impregnated carbon cloth translatesdirectly to a reduction in the thickness of the electrode structure,when assembled, and to a reduction in the resistance of the electrode1014. Further, and more importantly, application of the pressure to theimpregnated carbon cloth strips smooths the sprayed side of the carboncloth 1008 (smooths out the valleys and peaks) so that more surface areaof the sprayed aluminum layer 1012 is able to contact the paddle portion1002 of the current collector foils, so as to reduce the contactresistance R_(c) of the assembled capacitor. The pressing further helpswhen wrapping the electrodes as described below. Alternatively, manyelectrodes 1014 could be stacked and pressed at once; thus, saving time.

Referring back to FIG. 22A, once the electrodes have been formed, acontiguous porous separator sheet is provided to be used in forming theelectrode stack or winding assembly, indicated by block 2208.

The contiguous porous separator sheet is made from a suitableinsulator/separator based of material, such as polypropylene-basedseparator or polyethylene-based separator. The separator is typically apolypropylene-based material that is approximately 0.001 inches thick,and has an average pore size of about 0.04×0.12 μm. The separatormaterial used is a roll of material about 5.6 inches wide, enough toslightly overlap the paddle portion 1002 of the electrode 1014, andabout 12 feet in unrolled length.

In block 2214 of FIG. 22A, the wrapped electrode stack or windingassembly is formed. FIGS. 11-13 illustrate how the winding assembly isformed.

Referring to FIG. 11, a side sectional view of an embodiment of awrapped electrode stack or winding assembly comprising the electrodes ofFIGS. 10A-10E is shown. The wrapped electrode stack or winding assembly1100 has a contiguous porous separator sheet 1102, and a plurality ofelectrodes 1104 having a tab portion 1106 extending therefrom.

Fifty electrodes, for example, as shown in FIGS. 10A-10E are stackedsuch that the orientation of the tab portions 1106 is reversed for eachadjacent electrode 1104. Thus, the tab portions 1106 of all of theelectrodes 1104 of the stack extend in the same direction, but two setsof tab portions are formed. The orientation of the tab portions 1106 isnot shown in FIG. 11, but is reflected in FIG. 13. Every other tabportion is in one set of tab portions and the remaining every other tabportions are in the other set of tab portions. The two sets of tabportions connect every other electrode in parallel and; thus, willbecome the positive and negative terminals of the capacitor.

The electrodes 1104 are stacked such that a contiguous porous separatorsheet 1102 is interweaved between each electrode 1104 of the stack. Theporous separator sheet 1102 electrically insulates adjacent electrodes1104 to prevent electrically shorting against each other. The porousseparator sheet 1102 overlaps the impregnated carbon cloth strips byapproximately 0.250 inches; thus, the separator sheet 1102 extends 0.250inches over both ends of the electrode (see FIG. 13). The overlapensures the electrodes 1104 will be completely insulated from eachother.

The wrapped electrode stack or winding assembly 1100 is formed bystacking the electrodes and interweaving the separate sheet 1102 in sucha way that a first electrode is placed upon the porous separator sheet1102, then the separator sheet 1102 is folded over the first electrode.Next, a second electrode having its tab portion offset from the firstelectrode is placed on top of the covered first electrode and theseparator sheet 1102 is folded over. Then, a third electrode is stackedon top of the covered second electrode, the third electrode having thesame tab portion orientation as the first electrode. The separator sheet1102 is folded over the third electrode. The process repeats until thedesired number of electrodes have all been used. In this embodiment, 50impregnated carbon cloth electrodes 1104 are used.

Alternatively, the separator sheet 1102 may be cut into pieces that areplaced in between the adjacent electrodes. Then, the electrode stack iswrapped with a separator sheet 1102.

In another aspect of the present invention, the capacitor design lendsitself to multi-electrode scale up or scale down in order to meet theneeds of a particular double layer capacitor application. Thus, bysimply increasing or decreasing the size and number of compositeelectrodes that are used within the electrode stack, and by makingappropriate scaled changes in the physical parameters (size, weight,volume) of the capacitor (see FIGS. 16A-21B), it is possible to providea high performance double layer capacitor that is tailored to a specificapplication. With such a capacitor, the door is thus opened to a widevariety of applications wherein relatively large amounts of energy mustbe stored and retrieved from a compact storage device in a relativelyshort period of time.

Note that the bottom and top electrodes of the winding assembly 1100need only have impregnated carbon cloth placed against one side of thepaddle portion of the current collector foil. The impregnated carboncloth strips are only necessary on the side of the current collectorfoil facing the other electrodes. This cloth can be formed by cuttingthe impregnated cloth of FIG. 10A along the central fold line 1010.

Referring to FIG. 12, a front end sectional view of the wrappedelectrode stack or winding assembly 1100 of FIG. 11 is shown. Thewinding assembly 1200 has a plurality of impregnated carbon clothelectrodes 1202 having current collector foils 1206 and aluminumimpregnated carbon cloth strips 1204. The contiguous porous separatorsheet 1208 is shown.

From this view, the serpentine manner in which the contiguous separatorsheet 1208 winds throughout the winding assembly 1200 is betterillustrated. The contiguous separator sheet 1208 winds in between andaround adjacent electrodes 1202 in the winding assembly 1200; thus,electrically insulating the adjacent electrodes 1202 and the windingassembly 1200. Once the separator sheet 1208 winds through the entirestack, the separator sheet 1208 continues to wrap around the entireelectrode assembly, forming the winding assembly 1200.

Referring to FIG. 13, a top view of the embodiment shown in FIGS. 11 and12 is shown. The wrapped electrode stack or winding assembly 1300 isshown such that the orientation of the tab portions is illustrated, asearlier discussed with reference to FIGS. 11 and 12. For example, thetab portions of the first, third, fifth, etc. electrodes (starting fromthe bottom) are aligned in a first set of tab portions 1302. While thetab portions of the second, fourth, sixth, etc. electrodes are alignedin the second set of tab portions 1304. The two sets of tab portions1302 and 1304 are physically separated and electrically insulated fromeach other. Once the “prismatic” design double layer capacitor isfinished, the first set of tab portions 1302 will be coupled to onecapacitor terminal and the second set of tab portions 1304 will becoupled to another capacitor terminal. As will be seen in the FIGS. 18Athrough 19B, the first, third, fifth, etc., electrode will be connectedin parallel and the second, fourth, sixth, etc. electrodes will beconnected in parallel. Also shown is the porous separator sheet 1306 andthat it should overlap the paddle portion by approximately 0.250 inchesas shown.

Alternatively, the process of making the wrapped electrode stack orwinding assembly could be automated. The automated process uses reels ofimpregnated carbon cloth having the desired width, reels of aluminum tobe used for current collector foils having the desired width, and a reelof the contiguous porous separator sheet. As the reel of aluminum foilis unrolled, the current collector foils are cut to the shape shown inFIG. 10A. The reels of impregnated carbon cloth are unrolled and cutinto strips to be placed against the current collector foils as shown inFIGS. 10B-10E. As the electrodes are stacked, the reel containing theseparator sheet is unrolled and wrapped over and around the stack ofelectrodes by an arm that moves the separator sheet back and forth inthe serpentine manner shown in FIGS. 11-13.

Referring back to FIG. 22A, next the winding assembly is pressed (Block2216). The winding assembly is pressed in a mechanical press at 1800psi, for example. This pressing step may be in addition to previouslyhaving pressed the electrodes or may be the first time the electrodesare pressed. In addition to Reducing the contact resistance R_(c) of theassembled capacitor, the pressing makes it easier to wrap the windingassembly; thus, forming the brick assembly as described below.

Referring again back to FIG. 22A, the next step is to form the brickassembly, indicated in block 2218. The brick assembly is the wrappedelectrode stack or winding assembly having been tightly wrapped with aninsulating film and containing corner protectors and shims.

Referring to FIGS. 14A and 14B, shown are a side view and end view,respectively, of a brick assembly 1400 made from the winding assemblyshown in FIGS. 11 through 13. The winding assembly 1402 contains theelectrode stack wrapped in a contiguous porous separator sheet andhaving tab portions 1404 extending in the same direction as illustratedin FIG. 13. Also shown are respective shims 1406, insulating film 1408and 1409, and respective corner protectors 1410 having edges 1412.

For this embodiment of the present invention, the brick assembly 1400 isused to construct a “prismatic” design double layer capacitor, alsoreferred to as the PC 2500 design. The electrode stack is interweavedand wrapped with the contiguous porous separator sheet as shown abovewith reference to FIGS. 11-13. In order to add to the height of thewinding assembly 1402, a respective shim 1406 is placed on top andbottom of the winding assembly 1402. FIGS. 18B and 19B illustrate theposition of a shim 1406 from a top view. Once the brick assembly 1400 isinserted into a capacitor container, the shims 1406 will exert a modestconstant pressure against the winding assembly 1402 so that the exteriordimension (height) of the brick assembly 1400 is forced to conform withthe interior height of the capacitor container (see FIGS. 17 through18B). This modest constant pressure forces the aluminum impregnatedcarbon cloth strips into tight contact with the respective currentcollector foils, so that the collector resistance is minimized.

Briefly referring to FIG. 15, a 1500 shim is shown for use in forming abrick assembly. The shims 1500 are rectangular in shape having roundededges. The shim 1500 is made of a chemically neutral material , e.g.aluminum and is approximately 1.38 inches wide and 5.3 inches long. Thewidth of the shim 1500 varies depending on the interior dimensions ofthe capacitor container and the exterior dimension of the windingassembly. In this embodiment, the shim is 0.05 inches thick.

Referring back to FIG. 14, a shim 1406 is placed on top of the windingassembly 1402 and is held in place by tightly wrapping a layer ofpolyester insulating film 1408 around the winding assembly 1402 once.The insulating film 1408 (and 1409) is a chemically neutral insulator,such as polyester, having a width of about 1 inch and a thickness ofabout 0.005 inches. Typically, the film 1408 is tightly wrapped aroundthe winding assembly 1402 starting at the side opposite the tab portions1404 and wrapped in the direction of the end having the tab portions1404. The film 1408 should generously cover the ends of the windingassembly 1402. Next, another shim 1406 is placed on the bottom of theassembly and then tightly wrapped with another layer of polyester film1408.

After the shims 1406 are wrapped by the insulating film 1408, four Mylarcorner protectors 1410 are positioned around each corner, or lengthwiseedge, extending the length of the winding assembly 1402. The cornerprotectors 1410 are similar to the insulating film 1408, except thatthey comprise a thicker, more rigid film. The corner protectors are apiece of material approximately the length of the winding assembly 1402or slightly longer (here, about 5.4 inches) and wide enough to be foldedalong the edge of the assembly (here about 2 inches) overlapping theedge of the assembly as shown by the edge 1412 of the corner protector1410. The edge 1412 is shown as a dotted line since it is beneath alayer of film 1409. The corner protectors 1408 protect the corners andedges of the brick assembly 1400 during insertion of the brick assembly1400 into the capacitor container, so that the internal electrodes willnot be damaged, discussed further with reference to FIG. 17. Again,another layer of insulating film 1409 is tightly wrapped around theassembly to hold the Mylar corner protectors 1410 in place. Theresulting brick assembly 1400 has a fairly rigid shape, but it is stillcompressible from top to bottom. Once the polyester film 1408 iscompletely wrapped around the brick assembly 1400, the brick assembly1400 is tested for electrical shorts between the adjacent electrodes. Ifthe brick assembly 1400 passes, the brick assembly 1400 is ready toinserted into the capacitor container.

Referring to FIGS. 16A and 16B, a side view and a front view of thecapacitor container 1600 is shown. The capacitor container or “can” 1600for the prismatic design, PC 2500, is an elongated, open-ended containerfor holding the brick assembly 1400 of FIG. 14. The can 1600 is about6.1 inches long by 2.375 inches wide. It has rounded corners with aradius of 0.375 inches. The can 1600 is made of an annealed aluminumalloy and is about 0.043 inches thick.

Referring again to FIG. 22A, the brick assembly of FIGS. 14A and 14B isinserted into the can of FIGS. 16A and 16B, shown by block 2220.

Referring to FIG. 17, an illustration 1700 of the brick assembly beinginserted into the can is shown. Shown are the brick assembly 1704, thecan 1702, and two strips of Mylar corner protector material 1706 and1708. The two strips of corner protector material 1706 and 1708 arefolded about the brick assembly 1704 as shown. As the brick assembly1704 is pushed into the can 1702, the strips of corner protectormaterial 1706 and 1708 function like a “shoe horn” to guide the brickassembly 1704 into the can 1702 without damaging the internal componentsof the brick assembly 1704.

The strips of Mylar corner protector material 1706 and 1708, as well asthe corner protector material inside the brick assembly 1704, helpprotect the corners and edges of the brick assembly 1704 and ensure thatthe electrodes are not deformed upon insertion into the can 1702. Thecorners of the brick assembly 1704 are straight, while the corners ofthe can 1702 are rounded.

An important feature of the “prismatic” assembly is that the externaldimension (height) of the brick assembly 1704 is slightly greater thanthe interior dimension (height) of the can 1702. When the brick assembly1704 is inserted into the can 1702, a modest constant pressure isexerted on the brick assembly 1704. The importance of this feature isdiscussed with reference to FIG. 18A.

Referring again to FIG. 22A, once the brick assembly is fit into thecan, the capacitor terminals are formed in block 2222. FIGS. 18A-19Billustrate this process.

Referring to FIGS. 18A and 18B, a side view 1800 and a top view 1801,respectively, are shown of the brick assembly having been inserted intothe can. Shown are the can 1803, brick assembly 1802 having beencompressed, and the two sets of tab portions 1804 and 1806. The brickassembly 1802 contains the shims 1808, the corner protectors 1810 havingedges 1811, the polyester film 1812, and the folded strips of Mylar1814.

Shown in FIG. 18A, the tab portions of both sets of tab portions 1804and 1806 have all been folded down and level with the bottom of thebrick assembly 1802. The two sets of tab portions 1804 and 1806 arereally aligned, but are intentionally drawn to be misaligned forclarity. The edges 1811 of the corner protectors 1810 are indicated bydashed line.

FIG. 18B shows the positioning of the shim 1808 on the top side of thebrick assembly 1802 and the orientation of the sets of tab portions 1804and 1806 from the top view of the can 1803. A similar shim 1808 is onthe bottom side of the brick assembly 1802, as well. The two sets of tabportions 1804 and 1806 are more clearly illustrated. Again the tabportions of each set will be connected to a respective capacitorterminal. One set of tab portions (e.g., 1804) will be a positivepolarity and the other set of tab portions (e.g., 1806) will be anegative polarity.

FIG. 18A also illustrates the constant modest pressure that is placedupon the brick assembly 1802. The carbon cloth inside the brick assembly1802 is somewhat spongy, so that it is compressed sufficient to fitwithin the can 1803. The shims 1808 give the brick assembly 1802 justenough thickness on the top and bottom such that the interior of the can1803 puts pressure “P” on the brick assembly 1802. This continual modestpressure further serves to lower the contact and electrode resistance ofthe electrode assembly because it keeps the paddle portions of thecurrent collector foils in firm mechanical contact with the sprayed sideof the respective impregnated carbon cloth strips. The presence of suchconstant modest pressure is represented in the drawings by the arrowswhich symbolically represent that the brick assembly 1802 is maintainedunder a constant modest pressure, “P” applied in a direction so as toforce or press the electrodes in contact with the current collectorfoils. While the modest pressure is about 10 psi, in practice thepressure may vary anywhere from about 5 psi to 18 psi. The structuraldesign of the can 1803, while not comprising a pressure vessel per se,is nonetheless designed to withstand an internal pressure of up to about70 psi.

Referring to FIGS. 19A and 19B, the embodiment of the prismatic designof FIGS. 18A (1900) and 18B (1901) is shown having capacitor terminals1910 welded in position. Shown are the can 1902, the brick assembly1904, the two sets of tab portions 1906 and 1908, and two capacitorterminals 1910.

The two sets of tab portions 1906 and 1908 are formed by separating thetwo longest tab portions of each set of tab portions from the other tabportions. For example, the two longest tab portions (the bottom two tabportions) of the first set of tab portions 1906 are left laying flatwhile the remaining tab portions of the first set of tab portions 1906are folded back against the brick assembly 1904. The remaining tabportions (23 portions) are cut so that they are flush with the top edgeof the brick assembly 1904. The remaining tab portions are then foldedback down flat with the two longest tab portions. The two longest tabportions are cut about one inch (+/−0.125 inches is acceptable) longerthan the remaining tab portions and then folded over the remaining tabportions.

The other set of tab portions 1908 are formed using the same process,except the 23 separated tab portions are cut 1 inch (not greater than1.25 inches) away from the top edge of the brick assembly 1904 and thetwo longest tab portions are cut ¾ inch (+/−0.125 inches is acceptable)longer than the cut remaining tab portions having been folded back flat.Again, the two longest tab portions are folded over the 23 other tabportions.

The folded sets of tab portions 1906 and 1908 are then welded togetherand to a respective capacitor terminal 1910 using ultrasonic weldingsuch as shown in FIGS. 19C and 19D, or another suitable technique.

The capacitor terminals 1910 shown in FIGS. 19A and 19B as bonded suchthat the center of the capacitor terminal is about 0.83 inches from theedge of the brick assembly 1904. The centers of capacitor terminal 1910are located 0.97 inches from each other are in length and width.

Referring next to FIGS. 19C and 19D, a schematic representation is shownfor ultrasonically bonding many current collector foils is shown. Therepresentations 1920 and 1921 show a high frequency horn 1922 of the anultrasonic welder 1928, current collector foils 1924, and “dummy foils”1926.

Typically, ultrasonic welding is problematic for welding more than 8-10aluminum current collector foils (here, tab portions) together. Theproblem is that in order for the high frequency horn 1922 of theultrasonic welder 1928 to reach all of the layers of foil, the highfrequency horn 1922 cuts a pattern into several of the first layers ofaluminum foil; thus, reducing the effective transfer of current from thetop layers of current collector foils. Therefore, it is difficult toweld more than 10 layers of foil together without destroying thefunction of the top several foils as current collectors.

A solution is to provide “dummy foils” 1926 that sit on top of thecurrent collector foils 1924 to be bonded. As the high frequency horn1922 bonds the stack together, the high frequency horn 1922 cuts intothe “dummy foils” 1926 and not the current collector foils 1924. The“dummy foils” 1926 are ruined as far as functioning as a currentcollectors, but there is no harm since the “dummy foils” 1926 are notactually intended to collect current. By adding a variable number of“dummy foils” 1926 to the stack of foils to be bonded, the number ofcurrent collector foils 1924 that can be bonded is increased. In theembodiment shown in FIGS. 19A, 19B, and 19C, the two longest tabportions of each set of tab portions (current collector foils 1924) arefolded over act as the “dummy foils” 1926; thus, enabling all twentyfive current collector foils 1924 (tab portions) to be bonded togetherand to the respective capacitor terminals 1910 (not shown in FIGS. 19Cand 19D. Alternatively, as shown in FIG. 19D, several pieces of foilcould be cut and placed on top of the current collector foils 1924 toact as the “dummy foils” 1926 instead of folding over the two longesttab portions.

The technique for ultrasonically bonding a large number of currentcollector foils 1924 may have other applications than creating acapacitor terminal as shown in FIGS. 19A and 19B. The number of “dummyfoils” 1926 used will depend on the number of current collector foils1924 or metal foils used. The technique described enables at least 25current collector foils to be bonded to create a single electricalinterconnection.

Referring again to FIG. 22A, the next step is to install the capacitorterminals in the lid, block 2224. Then to close and seal the can to formthe “prismatic” design double layer capacitor, block 2226. These stepsare illustrated in FIGS. 20A through 21B.

Referring 20A and 20B, a front and side view, respectively, of the lidused to close and seal the prismatic design double layer capacitor isshown. The lid 2000 has two terminal holes 2002, one fill hole 2004, andpositive 2006 and negative 2008 markings. The terminal holes 2002 arefor receiving the capacitor terminals 1910 of FIGS. 19A and 19B. Thefill hole 2004 is for saturating the assembly with an electrolyticsolution, then receiving a fill plug (not shown). Typically, the lid2000 is comprised of the same material as the can.

Referring to FIGS. 21A and 21B, a side view and front end view,respectively, of the embodiment of the completed “prismatic” design, PC2500, is shown as assembled from the components and steps illustrated inFIGS. 10A through 20B. Shown are the can 2102, lid 2104, an O-ring seal2106, seal plug 2108, capacitor terminal 2110 (only one is shown), aretaining ring 2112, an insulating washer 2114, capacitor terminal seal2116, an insulator 2118, tubing 2120 and end insulator 2122, andelectrolyte solution 2124 (added later).

The capacitor terminals 2110 are installed onto the lid 2104. Aretaining ring 2112 and insulating washer 2114 are placed between theexternal edge of the capacitor terminal 2110 and the lid 2104 forholding the capacitor terminal 2110 firmly and for electricallyinsulating the lid 2104 and can 2102 from the capacitor terminal 2110.The lid 2102 is then placed into the can 2102 such that the sets of tabportions are bent as shown in order to seal the can 2102. A die cutTeflon insulator 2118 is placed between the lid 2104 and the brickassembly to further insulate the brick assembly. The capacitor terminalseal 2116 is used to seal the capacitor terminal 2110 to the lid 2104.Furthermore, an end insulator 2122 is placed at the end of the can 2102,then the assembly 2100 is layered with a tubing 2120. The tubing isclear PVC that is heat shrunk around the assembly 2100.

An important component needed to complete the capacitor assembly 2100 isa means for filling the closed assembly with a suitable electrolyticsolution 2124, and then permanently sealing the assembly 2100. To thisend a seal plug 2108, which is threadably received into a fill hole ofthe lid 2104 is provided. An O-ring seal 2106 is used with the seal plug2108 in order to effectuate the seal.

Referring again to FIG. 22A, once the capacitor assembly has been closed(block 2226), such as by welding the lid in place on the can, thecapacitor assembly is tested for electrical shorts. This test isperformed simply by measuring the resistance between the two capacitorterminals, each of which is conductive. In an ideal capacitor, thisresistance (for a “dry” assembly—no electrolyte yet introduced into theclosed case) should be infinite. A low resistance measurement, e.g., ofjust a few ohms, between the two terminals of the closed dry assembly,indicates that an electrical short has occurred internal to theassembly. In practice, a dry resistance of at least 20 MΩ is acceptableto pass this test for electrical shorts.

Referring now to FIG. 22B, the remaining steps are shown for testing andcompletion of the “prismatic” design double layer capacitor, PC 2500 inaccordance with one embodiment of present invention. Once the capacitorhas been assembled as shown in FIGS. 21A and 21B and tested forelectrical shorts (block 2228, FIG. 22A), the case assembly is sealed(block 2230), as required, or made sealable, using the seal plug andO-ring gasket. The sealable case assembly is then evacuated and theinternal components are thoroughly dried (block 2232). Such dryingprocess typically takes place over a 2 or 3 day period, and comprisesattaching a vacuum pump to the closed assembly, via the fill hole (FIGS.21A and 21B), and maintaining a constant negative pressure of about 10⁻⁶Torr for a specified period of time, e.g., 48 to 72 hours. Once dried,the capacitor assembly is tested for leaks (block 2234). Such leaktesting may be done using any suitable technique as is known in the art.A preferred leak test includes spraying an inert gas, e.g., helium (He),over and around the closed case assembly while it is still connected tothe vacuum pump, and while a negative pressure is still maintainedwithin it. If there is a leak, the negative pressure inside the caseassembly sucks the He gas through the leak, and the He gas can then bedetected in the outstream flow of the vacuum pump.

If the leakage test is successfully passed, then the case is ready to beimpregnated, through the fill hole, with a prescribed amount of aspecified electrolytic solution (block 2236).

The electrolytic solution is mixed by dissolving a selected salt in aprescribed solvent. Hence, to prepare the solution, the solvent isprepared (block 2238) and the specified salt (block 2240) is procured.As previously indicated, the preferred solvent is an organic solventacetonitrile (CH₃CN). The preferred salt is tetraethylammoniumtetraflouraborate, or (CH₃CH₂)₄N⁺BF₄ ⁻. Another preferred salt istriethylmethylammonium tetraflouraborate, or (CH₃CH₂)₃CH₂N⁺BF₄ ⁻. Othersalts, such as Imidizolium based salts may be used by the skilledartisan. The electrolytic solution is mixed (block 2242) by first dryingthe salt for at least 12 hours, and then dissolving the dried salt inthe solvent. The ratio of salt to solvent is 303.8 g/liter, which yields1.4 moles/liter.

Once mixed, the electrolyte is tested for impurities (block 2244). It isimportant that the amount of water in the electrolyte be reduced to lessthan 30 ppm (parts per million), preferably less than about 15 ppm. Ifthe level of impurities, e.g., water, in the electrolyte exceeds 30 ppm,the operating voltage of the capacitor may be adversely affected. Forexample, when the amount of water in the electrolyte reaches a level of40 ppm, the useful operating voltage of the capacitor is reduced toabout 70% of what it is when the water in the electrolyte is only 14ppm, as shown in FIGS. 23A and 23B. It is thus seen that it is importantfor impurities, particularly water, to be removed from the electrolytebefore the electrolyte is impregnated into the closed case assembly. (Itis noted that some additives may be added to the electrolyte, e.g., toenhance its performance or improve the operating life of the capacitor;but water must be avoided.)

The water content of the solution is measured using a coulometrictitrator, as is known in the art. A representative titrator that may beused for this purpose is the LC3000 Titrator available from EM ScienceAquastar.

Unfortunately, some water may already be inside of the closed caseassembly, despite attempts to thoroughly dry the inside of the assembly.For example, water may be trapped in the carbon fibers of the carboncloth. Such trapped water may be released into the electrolyte, therebybecoming an impurity within the electrolyte, as soon as theimpurity-free electrolyte is impregnated into the case assembly. Toremove such water (or similar impurities) from the carbon, it iscontemplated that the closed assembly be flushed with a suitablesolvent, e.g., acetonitrile, the electrolytic solution, or otherwater-scavenger material, prior to filling the assembly with theelectrolyte. It is also contemplated that the carbon cloth, prior tobeing impregnated with aluminum, and/or after being impregnated withaluminum, but before being assembled in the electrode stacks, may alsobe flushed or cleansed with a suitable material (e.g., water scavengersor additives that search out and remove water) selected to removeimpurities, especially water.

If the electrolytic solution successfully passes the impurity test(block 2244), it is also tested for conductivity (block 2246). Theconductivity test is performed using a conventional conductance meterthat measures conductance using an ac signal. The conductance of thesolution should be at least 55-58 mmho/cm at 22° C.

Once the electrolytic solution has been mixed and tested for impuritiesand conductivity, it is impregnated into the closed case assembly (block2236). Impregnation is preferably done by vacuum backfilling, as knownin the art. The amount of electrolytic solution that should beimpregnated into the closed case, for the “prismatic” case design shownin FIGS. 21A and 21B, is 280 g (322 ml).

After the prescribed amount of electrolytic solution has beenimpregnated into the closed case, the seal plug is inserted into thefill hole of the lid to finally seal the case (block 2248). Then, finalelectrical tests of the capacitor are performed (block 2250) to testwhether the capacitor meets its specified performance criteria.

Clamshell and Double-ended Packaging Designs

Alternative designs are possible for the single cell, multi-electrodedouble layer capacitor. The following embodiments parallel the processused to make the “prismatic design” double layer capacitor with severaldistinctions. FIGS. 22A and 22B, and other appropriate Figures, will bereferred to with reference to the following embodiments of the presentinvention.

Thus, turning to FIGS. 28A through 28B and FIGS. 24A through 27C, thebasic technique used in making a double layer capacitor in accordancewith this embodiment of the present invention will be described. FIGS.28A and 28B are a flow chart that illustrates the main steps in suchprocess; while FIGS. 24A through 27C illustrate individual steps of theprocess. Hence, in the description of the assembly and fabricationprocess that follows, reference will be made to specific blocks or boxesof the flow chart of FIGS. 28A and 28B to identify particular steps, atthe same time that reference is made to respective ones of FIGS. 24Athrough 27C to illustrate the step being carried out. FIGS. 28A through28B are similar to FIGS. 22A through 28B with several distinctions.Alternatively, the process used in FIGS. 28A through 28B may use thesame steps in FIGS. 22A through 22B.

With reference first to block 200 of FIG. 28A, an initial step to becarried out in making another embodiment of the capacitor 90 (FIG. 6) inaccordance with the present invention is to arc spray a suitable carboncloth with molten aluminum spray so that the aluminum is impregnateddeep into the tow of the fibers of the carbon cloth, just as is done inblock 2204 of FIG. 22A, and shown in FIGS. 8A through 9D.

Returning to FIG. 28A, it is seen that after the carbon cloth has beensprayed and impregnated with aluminum (block 200), the impregnatedcarbon cloth is precut into strips having dimensions greater than 2 by10 inches (block 202). The precut impregnated carbon cloth strips andthen die cut (block 204) to more exact dimensions of 2×10 inches, andthe corners of the strip are rounded to have a radius of approximately0.03 inches. The die cut impregnated carbon cloth strips are thenpressed in a mechanical press so as to be subjected to a pressure ofabout 1600 psi. The pressing step is further explained in connectionwith 2214 of FIG. 22A.

Still with reference to FIG. 28A, in a parallel path to preparing theimpregnated carbon cloth strips, the foil current collectors are alsoprepared. A first step in preparing the foil current collectors is toprecut aluminum foil to an approximate desired dimension (block 208),and then die cut the aluminum foil to the precise dimension (block 210).The preferred aluminum foil used for the current collector has athickness of approximately 0.002 inches. The foil is cut to a shapesubstantially as shown in FIG. 24A which is very similar to the currentcollector foil of FIG. 10A, except that the tab portion is not offsetfrom a central axis of the paddle portion. Such shape includes a paddleend 132 and a tab end 133. The tab end 133 and the paddle end 132 thuscomprise a current collector foil 130 (sometimes referred to as thecurrent collector plate). The current collector foil 130 is about teninches long. The paddle end 132 is about 6 inches long, and the tab end133 is about 4 inches long. The paddle end 132 has a width of about 2inches, and the tab end has a width of about 1 inch.

Two stacks of 25 current collector foils are next assembled (block 212,FIG. 28A) in the manner illustrated in FIG. 24B. In each stack, the tabends 133 of the twenty-five collector foils 130 are bonded together,using any suitable bonding technique, such as sintering or ultrasonicwelding, thereby forming a solid tab end 135 where each collector foilis thus electrically and mechanically connected in a secure manner toeach of the other collector foils in the stack. In contrast, the paddleends 132 of the collector foils 130 in the stack remain disconnectedfrom the other paddle ends. Note, that “dummy foils” as discussed withreference to FIGS. 19C and 19D may be used during the ultrasonic weldingprocess.

Referring back to FIG. 28A for the moment, it is seen that in additionto preparing the impregnated carbon cloths (blocks 200-206), andpreparing the aluminum current collector foils 130 (blocks 208-212),insulator sleeves 140 (FIG. 24F) must also be prepared. Such insulatorsleeves 140 function as the separator 66 (FIG. 4) in the double layercapacitor. The sleeves are made by precutting a suitableinsulator/separator material (block 214, FIG. 28A), such aspolypropylene or polyethylene, into strips. A suitable material for useas the separator is used, such as, Celguard 2400, as earlier described.The Celguard (or other separator material) is formed into sleeves ortubes (block 216, FIG. 28A) having a size that allows the sleeves toloosely slide over a current collector foil 130 which has an impregnatedcarbon cloth strip 136 folded around it, as shown in FIG. 24F. The edgesof the Celguard may be securely bonded to each other in order to formthe sleeve through use of any suitable sealing technique, such asthermal bonding, as is known in the art.

Once the current collector foils 130, the aluminum-impregnated carboncloth strips 136, and the separator sleeves 140 have been formed orotherwise fabricated, an electrode package may be assembled (block 218,FIG. 28A). Such electrode package assembly involves wrapping orsurrounding each of the foil paddles 132 of each electrode stack withthe impregnated carbon cloth strips 136 in the manner illustrated inFIGS. 24C, 24D and 24E. As seen in these figures, the cloth strips 136are folded at a central fold line 137, with the sprayed side of thecloth being placed against both sides of the paddle end 132 of thecollector foils 130. Each collector foils in each of the two collectorfoil stacks has a folded cloth strip 136 placed over it in this manner,except for the topmost collector foil in one stack, and the bottommostcollector foil in the other stack, which foils have a half of a clothstrip 136 positioned on the side of the collector foil that faces inwardin the stack. The separator sleeves 140 are then placed over thecombination of the carbon cloth strip 136 and the paddle end 132 of eachof the collector foils 130 of one of the two collector foil stacks,e.g., Stack “B”. The “leaves” of the two foil stacks (where a “leaf”comprises the collector foil and its accompanying carbon cloth strip),one having an separator/insulator sleeve 140 inserted over each leaf,and the other having no separator/insulator sleeve, are then interleavedwith each other as depicted in FIG. 25A to form an interleaved electrodeassembly 141. Alternatively, the separator sleeves 140 may simplycomprise a piece of separator material folded against both sides of thepaddle portions of Stack “B”.

The completed electrode assembly 141 includes a flat stack ofelectrodes, e.g., 50 electrodes. Each electrode is made up of a currentcollector foil 130 that is surrounded by an aluminum-impregnated carboncloth strip 136. Each carbon cloth strip is separated and electricallyinsulated from an adjacent carbon cloth stip by the separator material140. Alternating electrodes are electrically connected in parallel bythe bonded tabs 135 (Stack A) or 142 (Stack B) of the respective currentcollector foils.

An alternative method of assembling the electrode assembly 141 is verysimilar to the process of assembling the winding assembly 1200 of FIG.12. In this case, the electrodes formed in FIG. 24E are stacked suchthat the tab portions 133 generally face the opposite direction foradjacent electrodes. A contiguous separator sheet of Celguard is wrappedaround and between each electrode and then wrapped around the electrodestack. The separator sheet wraps throughout the stack in the sameserpentine manner as shown in FIGS. 11 and 12, except that the tabportions extend from opposite ends of electrode stack. Using thismethod, no separator sleeves are formed, since the separator sheetadequately insulates the adjacent electrodes. The tab portions 133 couldbe bonded together prior to winding the separator sheet or could bebonded together after the electrodes are stacked and wrapped. Thisalternative method is preferred since it eliminates the need toconstruct separator sleeves and eliminates the step of placing theseparator sleeve over each electrode. Furthermore, this method is moreconducive to an automated assembly procedure discussed below.

The assembly could also be automated as discussed with reference to FIG.13. In this method, a reel of aluminum current collector foil materialis unwound to be cut into current collector foils. A reel of impregnatedcarbon cloth is unwound and cut into strips which are then placed overthe current collector foils. And a reel is unwound that contains theseparator sheet is then folded over each electrode as it is stacked ontop of the stack with the tab portion extending the opposite directionas the previous electrode. The stack is then wrapped one time with theseparator material. The wrapped stack has a similar appearance as thatshown in FIGS. 12 or 25B.

An alternate electrode assembly 141′ that may be used in a spiral-woundembodiment of the invention is depicted in FIG. 25C. In FIG. 25C, twoelongate current collector foils 136′, each having a tab portion 133′that is connected to the appropriate capacitor terminals 70 and 74, andeach having a corresponding elongate aluminum-impregnated carbon cloth136′ folded over it so that a sprayed side 138′ of the cloth faces thefoil 132′, are spirally wound together. An insulator or separator sleeve140′ is placed over one of the foil/cloth electrodes of the woundassembly to prevent the electrodes from electrically shorting each otheras they are wound together.

The length and width of the current collector foils 132′ and thecorresponding aluminum-impregnated carbon cloth electrodes 136′ of thespiral-wound electrode assembly 141′ embodiment shown in FIG. 25C may bechosen so that approximately the same electrode area is achieved as isachieved using the interleaved flat stack assembly 141 shown in FIG.25A, or to achieve a desired performance criteria. An advantage of thespiral-wound assembly 141′ is that it is somewhat easier to assemble andmanufacture than the interleaved flat stack assembly 141. An advantageof the interleaved flat stack assembly 141, however, is that theresistance of the current collector foils may be lower (because it usesmany parallel short current collectors as opposed to one long currentcollector). Additionally, the interleaved flat stack assembly 141 lendsitself to more efficient use in a rectangular-shaped case, whereas thespiral-wound assembly 141′ is best suited for use in cylindrical-shapedcase. Depending on the application for which the capacitor is to beused, a rectangular-shaped case may prove more beneficial than acylindrical-shaped case.

Returning to a description of the assembly of the interleaved flat stackassembly 141 (FIG. 25A), after the two electrode stacks have beeninterleaved to form the assembly 141, the entire assembly is wrapped ina suitable insulating material 144, such as Celguard. The insulatingmaterial 144 may be held in place with a suitable tape 146 which is alsotightly wrapped around the assembly 141, thereby forming a wrapped flatstack electrode package 143. The current collector tabs 135 and 142extend from each end of the package 143.

Once the flat stack electrode package 144 has been fabricated, the finalmechanical assembly of the capacitor may be completed. Such mechanicalassembly is illustrated in FIG. 26, which figure shows an exploded viewof the physical components of the preferred double layer capacitor. Suchcomponents include a lower conductive shell 150 and an upper conductiveshell 154. One of the tabs, e.g., tab 135, of the electrode package 143is bonded to the inside of the lower shell 150 at location 160. Theother tab, e.g, tab 142, of the electrode package 143 is bonded to theinside of the upper shell 152 at a corresponding location. Such bonding(block 224, FIG. 28A) may be achieved using any suitable bondingtechnique, such as spot welding, ultrasonic welding, or the like. Thebond must, of course, be a low resistance bond, having a resistance ofno more than about 5 μΩ, if the overall low electrode resistance R_(EL),of the capacitor is to be maintained.

Once the tabs of the electrode package 143 have been bonded to therespective upper and lower conductive shells, the capacitor caseassembly is closed (block 226, FIG. 28A) by attaching and sealing theupper shell 152 to the lower shell 150 using any suitableattachment/sealing technique. Note that the upper and lower shells, incombination, comprise the case of the capacitor assembly. A preferredtechnique for closing the case of the capacitor, shown in FIG. 26, usesscrews 164, in combination with insulating nylon bushings 162, tosecurely fasten a flange 153 of the upper shell 152 to a correspondingflange 151 of the lower shell 150. To assure a good seal when theflanges of the upper and lower shells are joined together, an O-ring 154fits within a groove around the periphery of the flange 153, and anotherO-ring 156 fits within a similar groove around the periphery of theflange 151. Further, a polypropylene gasket 158 electrically insulatesthe two shells from each other.

Because, like clamshells, the case of the capacitor is closed byfastening the upper shell 152 fastened to the lower shell 150, thepackaging configuration depicted in FIGS. 24A through 26 is sometimesreferred to by the applicants as the “clamshell” assembly or the“clamshell” design.

An important feature of the “clam-shell” assembly shown in FIG. 26 isthat the electrode package 143, in its wrapped and interleaved form, hassomewhat larger dimensions than the inside dimensions of the upper andlower shells. However, because the carbon cloth is somewhat spongy, itis compressed sufficient to fit within the closed upper and lowershells. Hence, the package 143 remains slightly compressed as it placedinside of, and maintained within, the upper and lower shells. Thisresults in the electrode package 143 being maintained under a constantmodest pressure of about 10 psi when the upper shell 152 and the lowershell 150 are mechanically joined together. This continual modestpressure further serves to lower the contact and electrode resistance ofthe electrode assembly because it keeps the current collector foils 130in firm mechanical contact with the sprayed side of the respectiveimpregnated carbon cloth strips 136. The presence of such constantmodest pressure is represented in the drawings by the arrows 121 whichsymbolically represent that the electrode assembly 141 is maintainedunder a constant modest pressure, “P” applied in a direction so as toforce or press the electrodes in contact with the current collectorfoils (see FIG. 25B). Shims, such as those used in the prismatic designof FIGS. 8A through 21B may be used to apply the modest pressure aswell. For the spiral-wound assembly 141′, shown in FIG. 25C, theconstant modest pressure “P” is applied in a radial direction, asillustrated by the arrows 121′. While the modest pressure is about 10psi, in practice the pressure may vary anywhere from about 5 psi to 18psi. The structural design of the upper and lower shells (or othercapacitor case), while not comprising a pressure vessel per se, isnonetheless designed to withstand an internal pressure of up to about 20psi.

An important component needed to complete the capacitor assembly is ameans for filling the closed assembly with a suitable electrolyticsolution, and then permanently sealing the assembly. To this end a sealplug 168, which is threadably received into a fill hole 167 located atone end of the lower shell 150, is provided, as seen in FIG. 12. AnO-ring gasket 166 is used with the plug 168 is order to effectuate theseal. A similar fill hole (not shown) is located at the other end of theupper shell 152. Using two fill holes facilitates moving gases andfluids into and out of the closed assembly.

Referring again to FIG. 28A, once the case assembly has been closed(block 226), it is tested for electrical shorts. This test is performedsimply by measuring the resistance between the two shells (or halves ofthe case) in FIG. 26, each of which is conductive, function as theelectrical terminals of the capacitor. In an ideal capacitor, thisresistance (for a “dry” assembly—no electrolyte yet introduced into theclosed case) should be infinite. A low resistance measurement, e.g., ofjust a few ohms, between the upper and lower shells of the closed dryassembly, indicates that an electrical short has occurred internal tothe assembly. In practice, a dry resistance of at least 20 MΩ isacceptable to pass this test for electrical shorts.

Still with reference to FIG. 28A, it is noted that a step previouslyperformed before bonding the foil tabs to the case shells (block 224)comprises forming or otherwise fabricating the bottom shell 150 and thetop shell 152 (block 220). In the presently used embodiment, the shellsare each machined from a solid block of aluminum. The outside dimensionsof the closed assembly, including the flanges 151 and 153 are 2.25inches high by 2.62 inches wide and 5.60 inches long. The body of thecase (not including the flanges) has a width of about 2.18 inches, whichmeans the flanges 151 and 153 extend out from the body of the case about0.22 inches. The internal volume of the capacitor case is about 375 cm³,and the case weight is about 200 g.

As previously indicated, for the clamshell configuration shown in FIG.26, the upper and lower shells function as the two terminals of thecapacitor. It is contemplated that shells made using relativelyinexpensive stamped and/or pressed copper-clad aluminum, as opposed tomore-expensive machined aluminum blocks, may be used in the future.Copper-clad aluminum is preferred for this purpose, as opposed toaluminum, because it will provide a lower external contact resistancewhen several of the capacitors are stacked together. Using stampedand/or pressed materials to form the shells of the capacitor assemblyadvantageously reduces the weight of the case to about 100 g, andincreases the energy density from about 2.9 W-hr/kg to about 3.5W-hr/kg.

It should also be noted that alternative packaging schemes are alsocontemplated for the invention. For example, a double-ended capacitordesign, shown in FIGS. 27A, 27B and 27C, may be used. The double-endedconfiguration shown in FIGS. 27A, 27B and 27C includes an elongatedcapacitor case 170 having a generally square cross-section, that has aterminal 172 at each end of the package. The terminal 172 preferablyincludes a threaded hole 173 to which a threaded screw or bolt may beattached. The material of the case 170 may be conductive ornon-conductive. If conductive, the terminals are electrically insulatedfrom the case by the 176 and 178. The terminal 172 is attached to eachend of the double-ended assembly using a nut 174. A washer and/or gasket176 may be used with the nut 174 to firmly secure the terminal in placeand provide electrical insulation from the case when needed. Aninsulating gasket 178 is used on the inside of the case to seal theterminal 172 and prevent leaks. During assembly of the double-endeddesign, the tabs 135 and 142 of the flat stack internal electrodepackage 143 (FIG. 25B) are bonded to the inside of the terminals at eachend of the case 170.

Note that a seal plug 166 and gasket 168 are made available at at leastone end of the double ended capacitor, as shown in FIG. 27B. Preferably,a seal plug is made available in both ends of the capacitor tofacilitate filling the assembly with the electrolytic solution.

The main advantage of the double-ended configuration shown in FIGS. 27A,27B, and 27C is that the shell material need not be a conductor(although it can be), but may be a suitable light-weight non-conductivematerial, such as plastic. The overall weight of the case of thedouble-ended capacitor shown in FIGS. 27A, 27B and 27C may thus be madesignificantly less than the weight of the capacitor case for thecapacitor configuration shown in FIG. 26. The weight of the case isimportant because it contributes directly to the energy density of thecapacitor.

Because some alternative packaging schemes may include terminals, asillustrated above in connection with FIGS. 27A, 27B and 27C, the flowdiagram of FIG. 14A includes the step of installing the terminals on thecase, if such terminals are used (block 222).

Turning next to FIG. 28B, once the capacitor has been assembled as shownin FIG. 26 (or FIGS. 27A, 27B or 27C), and tested for electrical shorts(block 228, FIG. 28A), the case assembly is sealed (block 232), asrequired, or made sealable, using the seal plug 168 and gasket 166. Thesealable case is then evacuated and the internal components arethoroughly dried (block 234). Such drying process typically takes placeover a 2 or 3 day period, and comprises attaching a vacuum pump to theclosed assembly, via the fill hole 167 (FIG. 26), and maintaining aconstant negative pressure of about 10⁻⁶ Torr for a specified period oftime, e.g., 48 to 72 hours. Once dried, the assembly is tested for leaks(block 236). Such leak testing may be done using any suitable techniqueas is known in the art. A preferred leak test includes spraying an inertgas, e.g., helium (He), over and around the closed case while it isstill connected to the vacuum pump, and while a negative pressure isstill maintained within it. If there is a leak, the negative pressureinside the case sucks the He gas through the leak, and the He gas canthen be detected in the outstream flow of the vacuum pump.

If the leakage test is successfully passed, then the case is ready to beimpregnated, through the fill hole, with a prescribed amount of aspecified electrolytic solution (block 248).

The steps 238-246 in FIG. 28B are the same as those discussed in steps2238-2246 in FIG. 22B. See FIG. 22B for further details.

After the prescribed amount of electrolytic solution has beenimpregnated into the closed case, the plugs 168 are inserted into thefill holes 167 to finally seal the case (block 250; FIG. 28B). Then,final electrical tests of the capacitor are performed (block 262) totest whether the capacitor meets its specified performance criteria.

The final acceptance tests performed are detailed in Appendix A,attached hereto and incorporated herein by reference. (In this regard,it is noted that the flat stack clamshell capacitor design describedherein, and shown in FIG. 12, is referred to in Appendix A as the“UC3000”.) Generally, the acceptance tests include charging thecapacitor to is specified working voltage, V_(W), for six hours and thenallowing the capacitor to self-discharge over a fourteen hour period.The voltage drop that occurs during this 14 hour self-discharge periodprovides a measure of the equivalent parallel resistance of thecapacitor, which should be at least 200 ohms, preferably over 350-400 Ω,e.g., at least 360 Ω. (A self-discharge resistance of 200 Ω correspondsto a self-discharge time constant of at least 5.8 days.)

Additional acceptance tests that are performed include subjecting thecapacitor to a constant current cycle test to determine the cyclingcapacitance and steady state series resistance. This test is performedby applying a biphasic 100 amp and/or 200 amp current to the capacitoras shown in FIG. 29. The voltage waveform resulting from application ofthe current is measured. From the current and voltage waveforms, whichincludes time measurements, a large number of parameters are determinedto characterize the capacitor. Such parameters include the chargecapacitance, C_(up); the discharge capacitance, C_(down); the halfdischarge capacitance, C_(½), and the steady state resistance, R₀₀. Inorder to meet presently-imposed desired performance criteria, thesevalues should be C_(down)>2200 Farad, C_(½) 24 C_(down) by about 150Farad; R₀₀<1 milliohm, C_(up)/C_(down)>0.98; and C_(down)/C_(up)<1.05.

For the first group of single cell, multi-electrode double layercapacitors that have been made in accordance with the present invention,i.e., using the clamshell design shown in FIG. 26, the acceptance testdata is as shown in Table 3.

TABLE 3 Parameter Value Std. Deviation C_(down) 2422 f 44.6 f R_(∞)0.908 mΩ 0.058 mΩ C_(up)/C_(down) 1.01 R_(parallel) 387 Ω 53 Ω

The final acceptance tests also include ac impedance tests. Theextremely low impedance of the double layer capacitor makes the acimpedance measurements difficult using standard equipment andtechniques. The key parameter to measure is the initial resistance, R₀.This resistance affects the peak power the capacitor can deliver. It ismeasured at 1000 Hz using a Solatron 1250 Frequency Response Analyzerand a PARC 273 Poteniostat. R₀ should be about one-half of the value ofR₀₀, or about 0.45 mΩ.

At described above, it is thus seen that the single cell,multi-electrode double layer capacitor provided by the present inventionrepresents a significant advance in the double-layer capacitor art. Theuse of carbon cloth impregnated with aluminum, folded around a currentcollector foil plate, forms an efficient electrode structure thatprovides very low electrode resistance. By connecting a large number,e.g., twenty-five, of such electrodes in parallel in a first electrodestack, and interleaving the electrodes of the first electrode stack witha second electrode stack wherein each electrode is further surrounded bya suitable separator/insulator electrode sleeve, and then by packagingsuch interleaved electrode package within a sealed case that maintainsthe electrode package under a modest pressure, and then by furtherimpregnating the sealed case with a prescribed amount ofhighly-conductive non-aqueous electrolyte, a double layer capacitor isrealized that exhibits capacitance values in excess of 2200 Farad at anominal working voltage of about 2.3 volts, an electrode resistance ofabout 0.8 mΩ, a time constant of about 2 seconds, an energy density inthe range of 2.9-3.5 W-hr/kg, and a power rating of over 1000 W/kg at a400 Amp discharge. Advantageously, these operating parameters canimprove even more when the capacitor is operated at a higher voltage,e.g., 2.7 volts, or even 3.0 volts (which can be readily be done onceall the impurities are removed from the electrolytic solution) and theweight of the case is reduced. For example, at an operating voltage of3.0 volts, the energy density rises to 5.9 W-hr/kg. Further, by using apolyethylene separator material, instead of a polypropylene separator,the effective electrode resistance may be reduced even further, allowingthe time constant of the capacitor to be reduced to around 1.5 seconds.

While the invention described above has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those of skill in the artwithout departing from the scope of the invention set forth in theclaims.

What is claimed is:
 1. A method of ultrasonically bonding multiple foilstogether to form an electrical interconnection comprising; stacking tenor more metal foils to be bonded together, each of the ten or more metalfoils being coupled to an electrical device; positioning at least onedummy metal foil against the ten or more metal foils; and using a hornfor ultrasonically bonding the ten or more metal foils and the at leastone dummy metal foil together, the horn being directed at the at leastone dummy metal foil, wherein the ten or more metal foils remain intactand are bonded to each other and the at least one dummy metal foil. 2.The method of claim 1 wherein said positioning comprises saidpositioning said at least one dummy metal foil against said ten or moremetal foils, said at least one dummy metal foil comprises two or moredummy metal foils.
 3. The method of claim 1 wherein said positioningcomprises folding one or more of said ten or more metal foils over saidten or more metal foils such that a portion of the one or more of saidten or more metal foils form said at least one dummy metal foil, the oneor more of said ten or more metal foils having a length longer than alength of remaining ones of said ten or more metal foils.
 4. The methodof claim 1 wherein said stacking comprises said stacking ten or morealuminum foils.
 5. The method of claim 4 wherein said positioningcomprises positioning at least one dummy aluminum foil against said tenor more aluminum foils.
 6. A method of ultrasonically bonding multiplefoils together to form an electrical interconnection comprising:stacking a multiplicity of metal foils to be bonded together, each ofthe multiplicity of metal foils being coupled to an electrical device;positioning at least one dummy metal foil against the multiplicity ofmetal foils; and using a horn for ultrasonically bonding themultiplicity of metal foils and the at least one dummy metal foiltogether, the horn being directed at the at least one dummy metal foil,wherein the multiplicity of metal foils remain intact and are bonded toeach other and the at least one dummy metal foil.
 7. The method of claim6 wherein said stacking comprises said stacking said multiplicity ofmetal foils, said multiplicity of metal foils includes ten or more metalfoils.
 8. The method of claim 6 wherein said positioning comprises saidpositioning said at least one dummy metal foil against said multiplicityof metal foils, said at least one dummy metal foil comprises two or moredummy metal foils.
 9. The method of claim 6 wherein said positioningcomprises folding one or more of said multiplicity of metal foils oversaid multiplicity of metal foils such that a portion of the one or moreof said multiplicity of metal foils form said at least one dummy metalfoil, the one or more of said multiplicity of metal foils having alength longer than a length of remaining ones of said multiplicity ofmetal foils.
 10. The method of claim 6 wherein said stacking comprisessaid stacking a multiplicity of aluminum foils.
 11. The method of claim10 wherein said positioning comprises positioning at least one dummyaluminum foil against said multiplicity of aluminum foils.